In geometry, the semiperimeter of a polygon is half its perimeter. Although it has such a simple derivation from the perimeter, the semiperimeter appears frequently enough in formulas for triangles and other figures that it is given a separate name. When the semiperimeter occurs as part of a formula, it is typically denoted by the letter ''s''.

Triangles

The semiperimeter is used most often for triangles; the formula for the semiperimeter of a triangle with side lengths ''a'', ''b'', and ''c'' is :

s = \frac.

Properties

In any triangle, any vertex and the point where the opposite excircle touches the triangle partition the triangle's perimeter into two equal lengths, thus creating two paths each of which has a length equal to the semiperimeter. If A, B, C, A', B', and C' are as shown in the figure, then the segments connecting a vertex with the opposite excircle tangency (AA', BB', and CC', shown in red in the diagram) are known as

splitters Splitter or splitters may refer to: Technology * DSL filter or DSL splitter, in telecommunications * Fiber-optic splitter * Hybrid coil, a three windings transformer * Power dividers and directional couplers, in RF engineering * Siamese connect ...

, and

s = , AB, +, A'B, =, AB, +, AB', =, AC, +, A'C,

=, AC, +, AC', =, BC, +, B'C, =, BC, +, BC', .

The three splitters concur at the Nagel point of the triangle. A cleaver of a triangle is a line segment that bisects the perimeter of the triangle and has one endpoint at the midpoint of one of the three sides. So any cleaver, like any splitter, divides the triangle into two paths each of whose length equals the semiperimeter. The three cleavers concur at the center of the Spieker circle, which is the incircle of the medial triangle; the Spieker center is the

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

of all the points on the triangle's edges. A line through the triangle's incenter bisects the perimeter if and only if it also bisects the area. A triangle's semiperimeter equals the perimeter of its medial triangle. By the triangle inequality, the longest side length of a triangle is less than the semiperimeter.

Formulas invoking the semiperimeter

For triangles

The area ''A'' of any triangle is the product of its

inradius In geometry, the incircle or inscribed circle of a triangle is the largest circle that can be contained in the triangle; it touches (is tangent to) the three sides. The center of the incircle is a triangle center called the triangle's incenter. ...

(the radius of its inscribed circle) and its semiperimeter: :

A = rs.

The area of a triangle can also be calculated from its semiperimeter and side lengths ''a, b, c'' using Heron's formula: :

A = \sqrt.

The

circumradius In geometry, the circumscribed circle or circumcircle of a polygon is a circle that passes through all the vertices of the polygon. The center of this circle is called the circumcenter and its radius is called the circumradius. Not every polyg ...

''R'' of a triangle can also be calculated from the semiperimeter and side lengths: :

R = \frac .

This formula can be derived from the law of sines. The inradius is :

r = \sqrt.

The

law of cotangents In trigonometry, the law of cotangentsThe Universal Encyclopaedia of Mathematics, Pan Reference Books, 1976, page 530. English version George Allen and Unwin, 1964. Translated from the German version Meyers Rechenduden, 1960. is a relationship am ...

gives the

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 all ...

s of the half-angles at the vertices of a triangle in terms of the semiperimeter, the sides, and the inradius. The length of the internal bisector of the angle opposite the side of length ''a'' is :

t_a= \frac.

In a right triangle, the radius of the excircle on the hypotenuse equals the semiperimeter. The semiperimeter is the sum of the inradius and twice the circumradius. The area of the right triangle is

(s-a)(s-b)

where ''a'' and ''b'' are the legs.

For quadrilaterals

The formula for the semiperimeter of a quadrilateral with side lengths ''a'', ''b'', ''c'' and ''d'' is :

s = \frac.

One of the triangle area formulas involving the semiperimeter also applies to

tangential quadrilateral In Euclidean geometry, a tangential quadrilateral (sometimes just tangent quadrilateral) or circumscribed quadrilateral is a convex quadrilateral whose sides all can be tangent to a single circle within the quadrilateral. This circle is called the ...

s, which have an incircle and in which (according to

Pitot's theorem In geometry, the Pitot theorem, named after the French engineer Henri Pitot, states that in a tangential quadrilateral (i.e. one in which a circle can be inscribed) the two sums of lengths of opposite sides are the same. Both sums of lengths equa ...

) pairs of opposite sides have lengths summing to the semiperimeter—namely, the area is the product of the inradius and the semiperimeter: :

K = rs.

The simplest form of Brahmagupta's formula for the area of a cyclic quadrilateral has a form similar to that of Heron's formula for the triangle area: :

K = \sqrt.

Bretschneider's formula generalizes this to all convex quadrilaterals: :

K = \sqrt ,

in which

\alpha \,

and

\gamma \,

are two opposite angles. The four sides of a bicentric quadrilateral are the four solutions of a quartic equation parametrized by the semiperimeter, the inradius, and the circumradius.

Regular polygons

The area of a convex regular polygon is the product of its semiperimeter and its apothem.

References

External links

*{{mathworld , title = Semiperimeter , urlname = Semiperimeter Triangle geometry