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In mathematics, the inverse trigonometric functions (occasionally also called arcus functions, antitrigonometric functions or cyclometric functions) are the
inverse function In mathematics, the inverse function of a function (also called the inverse of ) is a function that undoes the operation of . The inverse of exists if and only if is bijective, and if it exists, is denoted by f^ . For a function f\colon ...
s of the
trigonometric functions In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions) are real functions which relate an angle of a right-angled triangle to ratios of two side lengths. They are widely used in ...
(with suitably restricted domains). Specifically, they are the inverses of the
sine In mathematics, sine and cosine are trigonometric functions of an angle. The sine and cosine of an acute angle are defined in the context of a right triangle: for the specified angle, its sine is the ratio of the length of the side that is opp ...
, cosine,
tangent In geometry, the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve. Mo ...
,
cotangent In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions) are real functions which relate an angle of a right-angled triangle to ratios of two side lengths. They are widely used in ...
, secant, and cosecant functions, and are used to obtain an angle from any of the angle's trigonometric ratios. Inverse trigonometric functions are widely used in
engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...
,
navigation Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation, ...
,
physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which rel ...
, and
geometry Geometry (; ) is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space such as the distance, shape, size, and relative position of figures. A mathematician who works in the field of geometry is c ...
.


Notation

Several notations for the inverse trigonometric functions exist. The most common convention is to name inverse trigonometric functions using an arc- prefix: , , , etc. (This convention is used throughout this article.) This notation arises from the following geometric relationships: when measuring in radians, an angle of ''θ'' radians will correspond to an arc whose length is ''rθ'', where ''r'' is the radius of the circle. Thus in the
unit circle In mathematics, a unit circle is a circle of unit radius—that is, a radius of 1. Frequently, especially in trigonometry, the unit circle is the circle of radius 1 centered at the origin (0, 0) in the Cartesian coordinate system in the Eucli ...
, "the arc whose cosine is ''x''" is the same as "the angle whose cosine is ''x''", because the length of the arc of the circle in radii is the same as the measurement of the angle in radians. In computer programming languages, the inverse trigonometric functions are often called by the abbreviated forms asin, acos, atan. The notations , , , etc., as introduced by
John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanic ...
in 1813, are often used as well in English-language sources, much more than the also established , , ,—conventions consistent with the notation of an
inverse function In mathematics, the inverse function of a function (also called the inverse of ) is a function that undoes the operation of . The inverse of exists if and only if is bijective, and if it exists, is denoted by f^ . For a function f\colon ...
, that is useful e.g. to define the multivalued version of each inverse trigonometric function: e.g. \tan^(x) = \. However, this might appear to conflict logically with the common semantics for expressions such as (although only , without parentheses, is the really common one), which refer to numeric power rather than function composition, and therefore may result in confusion between
multiplicative inverse In mathematics, a multiplicative inverse or reciprocal for a number ''x'', denoted by 1/''x'' or ''x''−1, is a number which when multiplied by ''x'' yields the multiplicative identity, 1. The multiplicative inverse of a fraction ''a''/''b ...
or reciprocal and compositional inverse. The confusion is somewhat mitigated by the fact that each of the reciprocal trigonometric functions has its own name—for example, = . Nevertheless, certain authors advise against using it for its ambiguity. Another precarious convention used by a tiny number of authors is to use an
uppercase Letter case is the distinction between the letters that are in larger uppercase or capitals (or more formally ''majuscule'') and smaller lowercase (or more formally ''minuscule'') in the written representation of certain languages. The writing ...
first letter, along with a superscript: , , , etc. Although its intention is to avoid confusion with the multiplicative inverse, which should be represented by , , etc., or, better, by , , etc., it in turn creates yet another major source of ambiguity: especially in light of the fact that many popular high-level programming languages (e.g. Wolfram's Mathematica, and University of Sydney's MAGMA) use those very same capitalised representations for the standard trig functions; whereas others (Python (ie SymPy and NumPy), Matlab, MAPLE etc) use lower-case. Hence, since 2009, the
ISO 80000-2 ISO 80000 or IEC 80000 is an international standard introducing the International System of Quantities (ISQ). It was developed and promulgated jointly by the International Organization for Standardization (ISO) and the International Electrotec ...
standard has specified solely the "arc" prefix for the inverse functions.


Basic concepts


Principal values

Since none of the six trigonometric functions are
one-to-one One-to-one or one to one may refer to: Mathematics and communication *One-to-one function, also called an injective function *One-to-one correspondence, also called a bijective function *One-to-one (communication), the act of an individual comm ...
, they must be restricted in order to have inverse functions. Therefore, the result ranges of the inverse functions are proper (i.e. strict)
subset In mathematics, set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset o ...
s of the domains of the original functions. For example, using in the sense of
multivalued function In mathematics, a multivalued function, also called multifunction, many-valued function, set-valued function, is similar to a function, but may associate several values to each input. More precisely, a multivalued function from a domain to ...
s, just as the
square root In mathematics, a square root of a number is a number such that ; in other words, a number whose ''square'' (the result of multiplying the number by itself, or  ⋅ ) is . For example, 4 and −4 are square roots of 16, because . ...
function y = \sqrt could be defined from y^2 = x, the function y = \arcsin(x) is defined so that \sin(y) = x. For a given real number x, with -1 \leq x \leq 1, there are multiple (in fact, countably infinitely many) numbers y such that \sin(y) = x; for example, \sin(0) = 0, but also \sin(\pi) = 0, \sin(2 \pi) = 0, etc. When only one value is desired, the function may be restricted to its principal branch. With this restriction, for each x in the domain, the expression \arcsin(x) will evaluate only to a single value, called its principal value. These properties apply to all the inverse trigonometric functions. The principal inverses are listed in the following table. Note: Some authors define the range of arcsecant to be ( 0 \leq y < \frac \text \pi \leq y < \frac ), because the tangent function is nonnegative on this domain. This makes some computations more consistent. For example, using this range, \tan(\arcsec(x)) = \sqrt, whereas with the range ( 0 \leq y < \frac \text \frac < y \leq \pi ), we would have to write \tan(\arcsec(x)) = \pm \sqrt, since tangent is nonnegative on 0 \leq y < \frac, but nonpositive on \frac < y \leq \pi. For a similar reason, the same authors define the range of arccosecant to be ( - \pi < y \leq - \frac or 0 < y \leq \frac ) . If x is allowed to be a
complex number In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the for ...
, then the range of y applies only to its real part.


Solutions to elementary trigonometric equations

Each of the trigonometric functions is periodic in the real part of its argument, running through all its values twice in each interval of 2 \pi: * Sine and cosecant begin their period at 2 \pi k-\frac (where k is an integer), finish it at 2 \pi k+\frac, and then reverse themselves over 2 \pi k+\frac, to 2 \pi k+\frac. * Cosine and secant begin their period at 2 \pi k, finish it at 2 \pi k+\pi. and then reverse themselves over 2 \pi k+\pi to 2 \pi k+2 \pi. * Tangent begins its period at 2 \pi k-\frac, finishes it at 2 \pi k+\frac, and then repeats it (forward) over 2 \pi k+\frac to 2 \pi k+\frac. * Cotangent begins its period at 2 \pi k, finishes it at 2 \pi k+\pi, and then repeats it (forward) over 2 \pi k+\pi to 2 \pi k+2 \pi. This periodicity is reflected in the general inverses, where k is some integer. The following table shows how inverse trigonometric functions may be used to solve equalities involving the six standard trigonometric functions. It is assumed that the given values \theta, r, s, x, and y all lie within appropriate ranges so that the relevant expressions below are
well-defined In mathematics, a well-defined expression or unambiguous expression is an expression whose definition assigns it a unique interpretation or value. Otherwise, the expression is said to be ''not well defined'', ill defined or ''ambiguous''. A fun ...
. Note that "for some k \in \Z" is just another way of saying "for some
integer An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the language ...
k." The symbol \,\iff\, is logical equality. The expression "LHS \,\iff\, RHS" indicates that (a) the left hand side (i.e. LHS) and right hand side (i.e. RHS) are true, or else (b) the left hand side and right hand side are false; there is option (c) (e.g. it is possible for the LHS statement to be true and also simultaneously for the RHS statement to false), because otherwise "LHS \,\iff\, RHS" would not have been written (see this footnoteTo clarify, suppose that it is written "LHS \,\iff\, RHS" where LHS (which abbreviates ''left hand side'') and RHS are both statements that can individually be either be true or false. For example, if \theta and s are some given and fixed numbers and if the following is written: \tan \theta = s \,\iff\, \theta = \arctan(s)+\pi k \quad \text k \in \Z then LHS is the statement "\tan \theta = s". Depending on what specific values \theta and s have, this LHS statement can either be true or false. For instance, LHS is true if \theta = 0 and s = 0 (because in this case \tan \theta = \tan 0 = s) but LHS is false if \theta = 0 and s = 2 (because in this case \tan \theta = \tan 0 = s which is not equal to s = 2); more generally, LHS is false if \theta = 0 and s \neq 0. Similarly, RHS is the statement "\theta = \arctan(s)+\pi k for some k \in \Z". The RHS statement can also either true or false (as before, whether the RHS statement is true or false depends on what specific values \theta and s have). The logical equality symbol \,\iff\, means that (a) if the LHS statement is true then the RHS statement is also true, and moreover (b) if the LHS statement is false then the RHS statement is also false. Similarly, \,\iff\, means that (c) if the RHS statement is true then the LHS statement is also true, and moreover (d) if the RHS statement is false then the LHS statement is also false. for an example illustrating this concept). For example, if \cos \theta = -1 then \theta = \pi+2 \pi k = -\pi+2 \pi (1+k) for some k \in \Z. While if \sin \theta = \pm 1 then \theta = \frac+\pi k =-\frac+\pi (k+1) for some k \in \Z, where k will be even if \sin \theta = 1 and it will be odd if \sin \theta = -1. The equations \sec \theta = -1 and \csc \theta = \pm 1 have the same solutions as \cos \theta = -1 and \sin \theta = \pm 1, respectively. In all equations above for those just solved (i.e. except for \sin/\csc \theta = \pm 1 and \cos/\sec \theta =-1), the integer k in the solution's formula is uniquely determined by \theta (for fixed r, s, x, and y). ;Detailed example and explanation of the "plus or minus" symbol \pm The solutions to \cos \theta = x and \sec \theta = x involve the "plus or minus" symbol \,\pm,\, whose meaning is now clarified. Only the solution to \cos \theta = x will be discussed since the discussion for \sec \theta = x is the same. We are given x between -1 \leq x \leq 1 and we know that there is an angle \theta in some interval that satisfies \cos \theta = x. We want to find this \theta. The table above indicates that the solution is \,\theta = \pm \arccos x+2 \pi k\, \quad \textk \in \Z which is a shorthand way of saying that (at least) one of the following statement is true:
#\,\theta = \arccos x+2 \pi k\, for some integer k,
or #\,\theta =-\arccos x+2 \pi k\, for some integer k. As mentioned above, if \,\arccos x = \pi\, (which by definition only happens when x = \cos \pi = -1) then both statements (1) and (2) hold, although with different values for the integer k: if K is the integer from statement (1), meaning that \theta = \pi+2 \pi K holds, then the integer k for statement (2) is K+1 (because \theta = -\pi+2 \pi (1+K)). However, if x \neq -1 then the integer k is unique and completely determined by \theta. If \,\arccos x = 0\, (which by definition only happens when x = \cos 0 = 1) then \,\pm\arccos x = 0\, (because \,+ \arccos x = +0 = 0\, and \,-\arccos x = -0 = 0\, so in both cases \,\pm \arccos x\, is equal to 0) and so the statements (1) and (2) happen to be identical in this particular case (and so both hold). Having considered the cases \,\arccos x = 0\, and \,\arccos x = \pi,\, we now focus on the case where \,\arccos x \neq 0\, and \,\arccos x \neq \pi,\, So assume this from now on. The solution to \cos \theta = x is still \,\theta = \pm \arccos x+2 \pi k\, \quad \textk \in \Z which as before is shorthand for saying that one of statements (1) and (2) is true. However this time, because \,\arccos x \neq 0\, and \,0 < \arccos x < \pi,\, statements (1) and (2) are different and furthermore, ''exactly one'' of the two equalities holds (not both). Additional information about \theta is needed to determine which one holds. For example, suppose that x = 0 and that that is known about \theta is that \,-\pi \leq \theta \leq \pi\, (and nothing more is known). Then \arccos x = \arccos 0 = \frac and moreover, in this particular case k = 0 (for both the \,+\, case and the \,-\, case) and so consequently, \theta ~=~ \pm \arccos x+2 \pi k ~=~ \pm \left(\frac\right)+2\pi (0) ~=~ \pm \frac. This means that \theta could be either \,\pi/2\, or \,-\pi/2. Without additional information it is not possible to determine which of these values \theta has. An example of some additional information that could determine the value of \theta would be knowing that the angle is above the x-axis (in which case \theta = \pi/2) or alternatively, knowing that it is below the x-axis (in which case \theta =-\pi/2). Transforming equations The equations above can be transformed by using the reflection and shift identities: These formulas imply, in particular, that the following hold: \begin \sin \theta &=-&&\sin(-\theta) &&=-&&\sin(\pi+\theta) &&= &&\sin(\pi-\theta) &&=-&&\cos\Big(\frac+\theta\Big) &&=\;&&\cos\Big(\frac-\theta\Big) &&=-&&\cos\Big(-\frac-\theta\Big) &&= &&\cos\Big(-\frac+\theta\Big) &&=-&&\cos\bigg(\frac-\theta\bigg) &&=-&&\cos\bigg(-\frac+\theta\bigg) \\ .3ex\cos \theta &= &&\cos(-\theta) &&=-&&\cos(\pi+\theta) &&= &&\cos(\pi-\theta) &&= &&\sin\Big(\frac+\theta\Big) &&= &&\sin\Big(\frac-\theta\Big) &&=-&&\sin\Big(-\frac-\theta\Big) &&=-&&\sin\Big(-\frac+\theta\Big) &&=-&&\sin\bigg(\frac-\theta\bigg) &&= &&\sin\bigg(-\frac+\theta\bigg) \\ .3ex\tan \theta &=-&&\tan(-\theta) &&= &&\tan(\pi+\theta) &&=-&&\tan(\pi-\theta) &&=-&&\cot\Big(\frac+\theta\Big) &&= &&\cot\Big(\frac-\theta\Big) &&= &&\cot\Big(-\frac-\theta\Big) &&=-&&\cot\Big(-\frac+\theta\Big) &&= &&\cot\bigg(\frac-\theta\bigg) &&=-&&\cot\bigg(-\frac+\theta\bigg) \\ .3ex\end where swapping \sin \leftrightarrow \csc, swapping \cos \leftrightarrow \sec, and swapping \tan \leftrightarrow \cot gives the analogous equations for \csc, \sec, \text \cot, respectively. So for example, by using the equality \sin \left(\frac-\theta\right) = \cos \theta, the equation \cos \theta = x can be transformed into \sin \left(\frac-\theta\right) = x, which allows for the solution to the equation \;\sin \varphi = x\; (where \varphi := \frac-\theta) to be used; that solution being: \varphi = (-1)^k \arcsin (x)+\pi k \; \text k \in \Z, which becomes: \frac-\theta ~=~ (-1)^k \arcsin (x)+\pi k \quad \text k \in \Z where using the fact that (-1)^ = (-1)^ and substituting h :=-k proves that another solution to \;\cos \theta = x\; is: \theta ~=~ (-1)^ \arcsin (x)+\pi h+\frac \quad \text h \in \Z. The substitution \;\arcsin x = \frac-\arccos x\; may be used express the right hand side of the above formula in terms of \;\arccos x\; instead of \;\arcsin x.\;


Equal identical trigonometric functions


Relationships between trigonometric functions and inverse trigonometric functions

Trigonometric functions of inverse trigonometric functions are tabulated below. A quick way to derive them is by considering the geometry of a right-angled triangle, with one side of length 1 and another side of length x, then applying the
Pythagorean theorem In mathematics, the Pythagorean theorem or Pythagoras' theorem is a fundamental relation in Euclidean geometry between the three sides of a right triangle. It states that the area of the square whose side is the hypotenuse (the side opposit ...
and definitions of the trigonometric ratios. Purely algebraic derivations are longer. It is worth noting that for arcsecant and arccosecant, the diagram assumes that x is positive, and thus the result has to be corrected through the use of absolute values and the signum (sgn) operation.


Relationships among the inverse trigonometric functions

Complementary angles: :\begin \arccos(x) &= \frac - \arcsin(x) \\ .5em\arccot(x) &= \frac - \arctan(x) \\ .5em\arccsc(x) &= \frac - \arcsec(x) \end Negative arguments: :\begin \arcsin(-x) &= -\arcsin(x) \\ \arccos(-x) &= \pi -\arccos(x) \\ \arctan(-x) &= -\arctan(x) \\ \arccot(-x) &= \pi -\arccot(x) \\ \arcsec(-x) &= \pi -\arcsec(x) \\ \arccsc(-x) &= -\arccsc(x) \end Reciprocal arguments: :\begin \arccos\left(\frac\right) &= \arcsec(x) \\ .3em\arcsin\left(\frac\right) &= \arccsc(x) \\ .3em\arctan\left(\frac\right) &= \frac - \arctan(x) = \arccot(x) \, , \text x > 0 \\ .3em\arctan\left(\frac\right) &= -\frac - \arctan(x) = \arccot(x) -\pi\, , \text x < 0 \\ .3em\arccot\left(\frac\right) &= \frac - \arccot(x) = \arctan(x) \, , \text x > 0 \\ .3em\arccot\left(\frac\right) &= \frac - \arccot(x) = \pi + \arctan(x) \, , \text x < 0 \\ .3em\arcsec\left(\frac\right) &= \arccos(x) \\ .3em\arccsc\left(\frac\right) &= \arcsin(x) \end Useful identities if one only has a fragment of a sine table: :\begin \arccos(x) &= \arcsin\left(\sqrt\right) \, , \text 0 \leq x \leq 1 \text \\ \arccos &\left(\frac\right) = \arcsin \left (\frac\right) \, , \text 0 \leq x \leq 1 \\ \arcsin &\left(\sqrt\right) =\frac-\sgn(x)\arcsin(x) \\ \arccos(x) &= \frac\arccos\left(2x^2-1\right) \, , \text 0 \leq x \leq 1 \\ \arcsin(x) &= \frac\arccos\left(1-2x^2\right) \, , \text 0 \leq x \leq 1 \\ \arcsin(x) &= \arctan\left(\frac\right) \\ \arccos(x) &= \arctan\left(\frac\right) \\ \arctan(x) &= \arcsin\left(\frac\right) \\ \arccot(x) &= \arccos\left(\frac\right) \end Whenever the square root of a complex number is used here, we choose the root with the positive real part (or positive imaginary part if the square was negative real). A useful form that follows directly from the table above is :\arctan\left(x\right) = \arccos\left(\sqrt\right)\, , \text x\geq 0 . It is obtained by recognizing that \cos\left(\arctan\left(x\right)\right) = \sqrt = \cos\left(\arccos\left(\sqrt\right)\right). From the half-angle formula, \tan\left(\tfrac\right) = \tfrac, we get: :\begin \arcsin(x) &= 2 \arctan\left(\frac\right) \\ .5em\arccos(x) &= 2 \arctan\left(\frac\right) \, , \text -1 < x \leq 1 \\ .5em\arctan(x) &= 2 \arctan\left(\frac\right) \end


Arctangent addition formula

:\arctan(u) \pm \arctan(v) = \arctan\left(\frac\right) \pmod \pi \, , \quad u v \ne 1 \, . This is derived from the tangent addition formula :\tan(\alpha \pm \beta) = \frac \, , by letting :\alpha = \arctan(u) \, , \quad \beta = \arctan(v) \, .


In calculus


Derivatives of inverse trigonometric functions

: The
derivative In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...
s for complex values of ''z'' are as follows: :\begin \frac \arcsin(z) & = \frac \; ; &z &\neq -1, +1 \\ \frac \arccos(z) & = -\frac \; ; &z &\neq -1, +1 \\ \frac \arctan(z) & = \frac \; ; &z &\neq -i, +i\\ \frac \arccot(z) & = -\frac \; ; &z &\neq -i, +i \\ \frac \arcsec(z) & = \frac \; ; &z &\neq -1, 0, +1 \\ \frac \arccsc(z) & = -\frac \; ; &z &\neq -1, 0, +1 \end Only for real values of ''x'': :\begin \frac \arcsec(x) & = \frac \; ; & , x, > 1\\ \frac \arccsc(x) & = -\frac \; ; & , x, > 1 \end For a sample derivation: if \theta = \arcsin(x), we get: :\frac = \frac = \frac = \frac = \frac = \frac


Expression as definite integrals

Integrating the derivative and fixing the value at one point gives an expression for the inverse trigonometric function as a definite integral: :\begin \arcsin(x) &= \int_0^x \frac \, dz \; , & , x, & \leq 1\\ \arccos(x) &= \int_x^1 \frac \, dz \; , & , x, & \leq 1\\ \arctan(x) &= \int_0^x \frac \, dz \; ,\\ \arccot(x) &= \int_x^\infty \frac \, dz \; ,\\ \arcsec(x) &= \int_1^x \frac \, dz = \pi + \int_^ \frac \, dz\; , & x & \geq 1\\ \arccsc(x) &= \int_x^\infty \frac \, dz = \int_^ \frac \, dz \; , & x & \geq 1\\ \end When ''x'' equals 1, the integrals with limited domains are improper integrals, but still well-defined.


Infinite series

Similar to the sine and cosine functions, the inverse trigonometric functions can also be calculated using
power series In mathematics, a power series (in one variable) is an infinite series of the form \sum_^\infty a_n \left(x - c\right)^n = a_0 + a_1 (x - c) + a_2 (x - c)^2 + \dots where ''an'' represents the coefficient of the ''n''th term and ''c'' is a con ...
, as follows. For arcsine, the series can be derived by expanding its derivative, \tfrac, as a binomial series, and integrating term by term (using the integral definition as above). The series for arctangent can similarly be derived by expanding its derivative \frac in a
geometric series In mathematics, a geometric series is the sum of an infinite number of terms that have a constant ratio between successive terms. For example, the series :\frac \,+\, \frac \,+\, \frac \,+\, \frac \,+\, \cdots is geometric, because each su ...
, and applying the integral definition above (see
Leibniz series Gottfried Wilhelm (von) Leibniz . ( – 14 November 1716) was a German polymath active as a mathematician, philosopher, scientist and diplomat. He is one of the most prominent figures in both the history of philosophy and the history of mathema ...
). : \begin \arcsin(z) & = z + \left( \frac \right) \frac + \left( \frac \right) \frac + \left( \frac \right) \frac + \cdots \\ pt& = \sum_^\infty \frac\frac \\ pt& = \sum_^\infty \frac \frac \, ; \qquad , z, \le 1 \end :\arctan(z) = z - \frac +\frac - \frac + \cdots = \sum_^\infty \frac \, ; \qquad , z, \le 1 \qquad z \neq i,-i Series for the other inverse trigonometric functions can be given in terms of these according to the relationships given above. For example, \arccos(x) = \pi/2 - \arcsin(x), \arccsc(x) = \arcsin(1/x), and so on. Another series is given by: :2\left(\arcsin\left(\frac\right) \right)^2 = \sum_^\infty \frac.
Leonhard Euler Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries in ma ...
found a series for the arctangent that converges more quickly than its
Taylor series In mathematics, the Taylor series or Taylor expansion of a function is an infinite sum of terms that are expressed in terms of the function's derivatives at a single point. For most common functions, the function and the sum of its Taylor se ...
: : \arctan(z) = \frac z \sum_^\infty \prod_^n \frac. (The term in the sum for ''n'' = 0 is the
empty product In mathematics, an empty product, or nullary product or vacuous product, is the result of multiplying no factors. It is by convention equal to the multiplicative identity (assuming there is an identity for the multiplication operation in questio ...
, so is 1.) Alternatively, this can be expressed as :\arctan(z) = \sum_^\infty \frac \frac. Another series for the arctangent function is given by :\arctan(z) = i\sum_^\infty\frac\left(\frac - \frac\right), where i=\sqrt is the
imaginary unit The imaginary unit or unit imaginary number () is a solution to the quadratic equation x^2+1=0. Although there is no real number with this property, can be used to extend the real numbers to what are called complex numbers, using addition a ...
.


Continued fractions for arctangent

Two alternatives to the power series for arctangent are these generalized continued fractions: : \arctan(z) = \frac z = \frac The second of these is valid in the cut complex plane. There are two cuts, from −i to the point at infinity, going down the imaginary axis, and from i to the point at infinity, going up the same axis. It works best for real numbers running from −1 to 1. The partial denominators are the odd natural numbers, and the partial numerators (after the first) are just (''nz'')2, with each perfect square appearing once. The first was developed by
Leonhard Euler Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries in ma ...
; the second by
Carl Friedrich Gauss Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refe ...
utilizing the Gaussian hypergeometric series.


Indefinite integrals of inverse trigonometric functions

For real and complex values of ''z'': :\begin \int \arcsin(z) \, dz &= z \, \arcsin(z) + \sqrt + C\\ \int \arccos(z) \, dz &= z \, \arccos(z) - \sqrt + C\\ \int \arctan(z) \, dz &= z \, \arctan(z) - \frac \ln \left( 1 + z^2 \right) + C\\ \int \arccot(z) \, dz &= z \, \arccot(z) + \frac \ln \left( 1 + z^2 \right) + C\\ \int \arcsec(z) \, dz &= z \, \arcsec(z) - \ln \left z \left( 1 + \sqrt \right) \right+ C\\ \int \arccsc(z) \, dz &= z \, \arccsc(z) + \ln \left z \left( 1 + \sqrt \right) \right+ C \end For real ''x'' ≥ 1: :\begin \int \arcsec(x) \, dx &= x \, \arcsec(x) - \ln \left( x + \sqrt \right) + C\\ \int \arccsc(x) \, dx &= x \, \arccsc(x) + \ln \left( x + \sqrt \right) + C \end For all real ''x'' not between -1 and 1: :\begin \int \arcsec(x) \, dx &= x \, \arcsec(x) - \sgn(x) \ln\left, x + \sqrt\ + C\\ \int \arccsc(x) \, dx &= x \, \arccsc(x) + \sgn(x) \ln\left, x + \sqrt\ + C \end The absolute value is necessary to compensate for both negative and positive values of the arcsecant and arccosecant functions. The signum function is also necessary due to the absolute values in the
derivative In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...
s of the two functions, which create two different solutions for positive and negative values of x. These can be further simplified using the logarithmic definitions of the inverse hyperbolic functions: :\begin \int \arcsec(x) \, dx &= x \, \arcsec(x) - \operatorname(, x, ) + C\\ \int \arccsc(x) \, dx &= x \, \arccsc(x) + \operatorname(, x, ) + C\\ \end The absolute value in the argument of the arcosh function creates a negative half of its graph, making it identical to the signum logarithmic function shown above. All of these antiderivatives can be derived using
integration by parts In calculus, and more generally in mathematical analysis, integration by parts or partial integration is a process that finds the integral of a product of functions in terms of the integral of the product of their derivative and antiderivative. ...
and the simple derivative forms shown above.


Example

Using \int u \, dv = u v - \int v \, du (i.e.
integration by parts In calculus, and more generally in mathematical analysis, integration by parts or partial integration is a process that finds the integral of a product of functions in terms of the integral of the product of their derivative and antiderivative. ...
), set :\begin u &= \arcsin(x) & dv &= dx \\ du &= \frac & v &= x \end Then :\int \arcsin(x) \, dx = x \arcsin(x) - \int \frac \, dx, which by the simple substitution w=1-x^2,\ dw = -2x\,dx yields the final result: :\int \arcsin(x) \, dx = x \arcsin(x) + \sqrt + C


Extension to complex plane

Since the inverse trigonometric functions are
analytic function In mathematics, an analytic function is a function that is locally given by a convergent power series. There exist both real analytic functions and complex analytic functions. Functions of each type are infinitely differentiable, but complex ...
s, they can be extended from the real line to the complex plane. This results in functions with multiple sheets and
branch point In the mathematical field of complex analysis, a branch point of a multi-valued function (usually referred to as a "multifunction" in the context of complex analysis) is a point such that if the function is n-valued (has n values) at that point, ...
s. One possible way of defining the extension is: :\arctan(z) = \int_0^z \frac \quad z \neq -i, +i where the part of the imaginary axis which does not lie strictly between the branch points (−i and +i) is the branch cut between the principal sheet and other sheets. The path of the integral must not cross a branch cut. For ''z'' not on a branch cut, a straight line path from 0 to ''z'' is such a path. For ''z'' on a branch cut, the path must approach from for the upper branch cut and from for the lower branch cut. The arcsine function may then be defined as: :\arcsin(z) = \arctan\left(\frac\right) \quad z \neq -1, +1 where (the square-root function has its cut along the negative real axis and) the part of the real axis which does not lie strictly between −1 and +1 is the branch cut between the principal sheet of arcsin and other sheets; :\arccos(z) = \frac - \arcsin(z) \quad z \neq -1, +1 which has the same cut as arcsin; :\arccot(z) = \frac - \arctan(z) \quad z \neq -i, i which has the same cut as arctan; :\arcsec(z) = \arccos\left(\frac\right) \quad z \neq -1, 0, +1 where the part of the real axis between −1 and +1 inclusive is the cut between the principal sheet of arcsec and other sheets; :\arccsc(z) = \arcsin\left(\frac\right) \quad z \neq -1, 0, +1 which has the same cut as arcsec.


Logarithmic forms

These functions may also be expressed using complex logarithms. This extends their domains to the
complex plane In mathematics, the complex plane is the plane formed by the complex numbers, with a Cartesian coordinate system such that the -axis, called the real axis, is formed by the real numbers, and the -axis, called the imaginary axis, is formed by th ...
in a natural fashion. The following identities for principal values of the functions hold everywhere that they are defined, even on their branch cuts. :\begin \arcsin(z) &= -i \ln \left( \sqrt + iz \right) = i \ln \left( \sqrt - iz \right) &= \arccsc\left(\frac\right) \\ 0pt\arccos(z) &= -i \ln \left( i \sqrt + z \right) = \frac - \arcsin(z) &= \arcsec\left(\frac\right) \\ 0pt\arctan(z) &= -\frac\ln \left(\frac\right) = -\frac\ln \left(\frac\right) &= \arccot\left(\frac\right) \\ 0pt\arccot(z) &= -\frac\ln\left( \frac \right) = -\frac\ln\left( \frac \right) &= \arctan\left(\frac\right) \\ 0pt\arcsec(z) &= -i \ln \left( i \sqrt + \frac \right) = \frac - \arccsc(z) &= \arccos\left(\frac\right) \\ 0pt\arccsc(z) &= -i \ln \left( \sqrt + \frac \right) = i \ln \left( \sqrt - \frac \right) &= \arcsin\left(\frac\right) \end


Generalization

Because all of the inverse trigonometric functions output an angle of a right triangle, they can be generalized by using Euler's formula to form a right triangle in the complex plane. Algebraically, this gives us: : ce^ = c\cos(\theta) + ic\sin(\theta) or : ce^ = a + ib where a is the adjacent side, b is the opposite side, and c is the hypotenuse. From here, we can solve for \theta. :\begin e^ & = a + ib \\ \ln c + i\theta & = \ln(a + ib) \\ \theta & = \operatorname\left( \ln(a + ib) \right) \end or : \theta = -i\ln\left( \frac \right) Simply taking the imaginary part works for any real-valued a and b, but if a or b is complex-valued, we have to use the final equation so that the real part of the result isn't excluded. Since the length of the hypotenuse doesn't change the angle, ignoring the real part of \ln(a+bi) also removes c from the equation. In the final equation, we see that the angle of the triangle in the complex plane can be found by inputting the lengths of each side. By setting one of the three sides equal to 1 and one of the remaining sides equal to our input z, we obtain a formula for one of the inverse trig functions, for a total of six equations. Because the inverse trig functions require only one input, we must put the final side of the triangle in terms of the other two using the
Pythagorean Theorem In mathematics, the Pythagorean theorem or Pythagoras' theorem is a fundamental relation in Euclidean geometry between the three sides of a right triangle. It states that the area of the square whose side is the hypotenuse (the side opposit ...
relation : a^2 + b^2 = c^2 The table below shows the values of a, b, and c for each of the inverse trig functions and the equivalent expressions for \theta that result from plugging the values into the equations above and simplifying. :\begin & a & & b & & c && -i\ln\left(\frac\right) && \theta && \theta_\\ \arcsin(z)\ \ & \sqrt & & z & & 1 & & -i\ln\left( \frac \right) && = -i\ln\left( \sqrt + iz \right) && \operatorname\left(\ln\left( \sqrt + iz \right)\right) \\ \arccos(z)\ \ & z & & \sqrt & & 1 & & -i\ln\left( \frac \right) && = -i\ln\left( z + \sqrt \right) && \operatorname\left(\ln\left( z + \sqrt \right)\right) \\ \arctan(z)\ \ & 1 & & z & & \sqrt & & -i\ln\left( \frac \right) && = -i\ln\left( \frac \right) && \operatorname\left(\ln\left( 1 + iz \right)\right) \\ \arccot(z)\ \ & z & & 1 & & \sqrt & & -i\ln\left( \frac \right) && = -i\ln\left( \frac \right) && \operatorname\left(\ln\left( z + i \right)\right) \\ \arcsec(z)\ \ & 1 & & \sqrt & & z & & -i\ln\left( \frac \right) && = -i\ln\left( \frac + \sqrt \right) && \operatorname\left(\ln\left( \frac + \sqrt \right)\right) \\ \arccsc(z)\ \ & \sqrt & & 1 & & z & & -i\ln\left( \frac \right) && = -i\ln\left( \sqrt + \frac \right) && \operatorname\left(\ln\left( \sqrt + \frac \right)\right) \\ \end In order to match the principal branch of the natural log and square root functions to the usual principal branch of the inverse trig functions, the particular form of the simplified formulation matters. The formulations given in the two rightmost columns assume \operatorname\left( \ln z \right) \in (-\pi,\pi] and \operatorname\left(\sqrt\right) \ge 0 . To match the principal branch \operatorname\left( \ln z \right) \in [0,2\pi) and \operatorname\left(\sqrt\right) \ge 0 to the usual principal branch of the inverse trig functions, subtract 2\pi from the result \theta when \operatorname(\theta)>\pi. In this sense, all of the inverse trig functions can be thought of as specific cases of the complex-valued log function. Since these definition work for any complex-valued z, the definitions allow for hyperbolic angles as outputs and can be used to further define the inverse hyperbolic functions. Elementary proofs of the relations may also proceed via expansion to exponential forms of the trigonometric functions.


Example proof

:\begin \sin(\phi) &= z \\ \phi &= \arcsin(z) \end Using the exponential definition of sine, and letting \xi = e^, :\begin z &= \frac \\ 0mu2iz &= \xi - \frac \\ mu0 &= \xi^2 - 2i z \xi - 1 \\ mu\xi &= iz \pm \sqrt \\ mu\phi &= -i \ln \left(iz \pm \sqrt\right) \end (the positive branch is chosen) :\phi= \arcsin(z) = -i \ln \left(iz + \sqrt \right)


Applications


Finding the angle of a right triangle

Inverse trigonometric functions are useful when trying to determine the remaining two angles of a
right triangle A right triangle (American English) or right-angled triangle ( British), or more formally an orthogonal triangle, formerly called a rectangled triangle ( grc, ὀρθόσγωνία, lit=upright angle), is a triangle in which one angle is a right ...
when the lengths of the sides of the triangle are known. Recalling the right-triangle definitions of sine and cosine, it follows that :\theta = \arcsin \left( \frac \right) = \arccos \left( \frac \right) . Often, the hypotenuse is unknown and would need to be calculated before using arcsine or arccosine using the
Pythagorean Theorem In mathematics, the Pythagorean theorem or Pythagoras' theorem is a fundamental relation in Euclidean geometry between the three sides of a right triangle. It states that the area of the square whose side is the hypotenuse (the side opposit ...
: a^2+b^2=h^2 where h is the length of the hypotenuse. Arctangent comes in handy in this situation, as the length of the hypotenuse is not needed. :\theta = \arctan \left( \frac \right) \, . For example, suppose a roof drops 8 feet as it runs out 20 feet. The roof makes an angle ''θ'' with the horizontal, where ''θ'' may be computed as follows: :\theta = \arctan \left( \frac \right) = \arctan \left( \frac \right) = \arctan \left( \frac \right) \approx 21.8^ \, .


In computer science and engineering


Two-argument variant of arctangent

The two-argument atan2 function computes the arctangent of ''y'' / ''x'' given ''y'' and ''x'', but with a range of (−, ]. In other words, atan2(''y'', ''x'') is the angle between the positive ''x''-axis of a plane and the point (''x'', ''y'') on it, with positive sign for counter-clockwise angles (upper half-plane, ''y'' > 0), and negative sign for clockwise angles (lower half-plane, ''y'' < 0). It was first introduced in many computer programming languages, but it is now also common in other fields of science and engineering. In terms of the standard arctan function, that is with range of (−, ), it can be expressed as follows: :\operatorname(y, x) = \begin \arctan\left(\frac y x\right) & \quad x > 0 \\ \arctan\left(\frac y x\right) + \pi & \quad y \ge 0,\; x < 0 \\ \arctan\left(\frac y x\right) - \pi & \quad y < 0,\; x < 0 \\ \frac & \quad y > 0,\; x = 0 \\ -\frac & \quad y < 0,\; x = 0 \\ \text & \quad y = 0,\; x = 0 \end It also equals the principal value of the arg (mathematics), argument of the
complex number In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the for ...
''x'' + i''y''. This limited version of the function above may also be defined using the tangent half-angle formulae as follows: :\operatorname(y, x) = 2\arctan\left(\frac\right) provided that either ''x'' > 0 or ''y'' ≠ 0. However this fails if given x ≤ 0 and y = 0 so the expression is unsuitable for computational use. The above argument order (''y'', ''x'') seems to be the most common, and in particular is used in ISO standards such as the
C programming language ''The C Programming Language'' (sometimes termed ''K&R'', after its authors' initials) is a computer programming book written by Brian Kernighan and Dennis Ritchie, the latter of whom originally designed and implemented the language, as well as ...
, but a few authors may use the opposite convention (''x'', ''y'') so some caution is warranted. These variations are detailed at atan2.


Arctangent function with location parameter

In many applicationswhen a time varying angle crossing \pm\pi/2 should be mapped by a smooth line instead of a saw toothed one (robotics, astromomy, angular movement in general) the solution y of the equation x=\tan(y) is to come as close as possible to a given value -\infty < \eta < \infty. The adequate solution is produced by the parameter modified arctangent function : y = \arctan_\eta(x) := \arctan(x) + \pi \, \operatorname\left(\frac \right)\, . The function \operatorname rounds to the nearest integer.


Numerical accuracy

For angles near 0 and , arccosine is ill-conditioned and will thus calculate the angle with reduced accuracy in a computer implementation (due to the limited number of digits). Similarly, arcsine is inaccurate for angles near −/2 and /2.


See also

* Arcsine distribution *
Inverse exsecant The exsecant (exsec, exs) and excosecant (excosec, excsc, exc) are trigonometric functions defined in terms of the secant and cosecant functions. They used to be important in fields such as surveying, railway engineering, civil engineering, astr ...
*
Inverse versine The versine or versed sine is a trigonometric function found in some of the earliest (Sanskrit ''Aryabhatia'',Inverse hyperbolic functions In mathematics, the inverse hyperbolic functions are the inverse functions of the hyperbolic functions. For a given value of a hyperbolic function, the corresponding inverse hyperbolic function provides the corresponding hyperbolic angle. The ...
* List of integrals of inverse trigonometric functions *
List of trigonometric identities In trigonometry, trigonometric identities are equalities that involve trigonometric functions and are true for every value of the occurring variables for which both sides of the equality are defined. Geometrically, these are identities involving ...
*
Trigonometric function In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions) are real functions which relate an angle of a right-angled triangle to ratios of two side lengths. They are widely used in ...
* Trigonometric functions of matrices


Notes


References

*


External links

* {{DEFAULTSORT:Inverse Trigonometric Functions Trigonometry Elementary special functions Mathematical relations Ratios Dimensionless numbers