Similarity (geometry)

In Euclidean geometry, two objects are similar if they have the same shape, or one has the same shape as the mirror image of the other. More precisely, one can be obtained from the other by uniformly scaling (enlarging or reducing), possibly with additional translation, rotation and reflection. This means that either object can be rescaled, repositioned, and reflected, so as to coincide precisely with the other object. If two objects are similar, each is congruent to the result of a particular uniform scaling of the other.

For example, all circles are similar to each other, all squares are similar to each other, and all equilateral triangles are similar to each other. On the other hand, ellipses are not all similar to each other, rectangles are not all similar to each other, and isosceles triangles are not all similar to each other.

If two angles of a triangle have measures equal to the measures of two angles of another triangle, then the triangles are similar. Corresponding sides of similar polygons are in proportion, and corresponding angles of similar polygons have the same measure.

Two congruent shapes are similar, with a scale factor of 1. However, some school textbooks specifically exclude congruent triangles from their definition of similar triangles by insisting that the sizes must be different if the triangles are to qualify as similar.

Similar triangles

Two triangles, and , are similar if and only if corresponding angles have the same measure: this implies that they are similar if and only if the lengths of corresponding sides are proportional. It can be shown that two triangles having congruent angles (''equiangular triangles'') are similar, that is, the corresponding sides can be proved to be proportional. This is known as the AAA similarity theorem. Note that the "AAA" is a mnemonic: each one of the three A's refers to an "angle". Due to this theorem, several authors simplify the definition of similar triangles to only require that the corresponding three angles are congruent.

There are several criteria each of which is necessary and sufficient for two triangles to be similar:

*Any two pairs of congruent angles, which in Euclidean geometry implies that their all three angles are congruent:

::If is equal in measure to , and is equal in measure to , then this implies that is equal in measure to and the triangles are similar.

*All the corresponding sides are proportional:

:: . This is equivalent to saying that one triangle (or its mirror image) is an enlargement of the other.

*Any two pairs of sides are proportional, and the angles included between these sides are congruent:
:: and is equal in measure to .
This is known as the SAS similarity criterion. The "SAS" is a mnemonic: each one of the two S's refers to a "side"; the A refers to an "angle" between the two sides.

Symbolically, we write the similarity and dissimilarity of two triangles and as follows:

:$ABC\sim A'B'C'$
:$ABC\nsim A'B'C'$

There are several elementary results concerning similar triangles in Euclidean geometry:
* Any two equilateral triangles are similar.
* Two triangles, both similar to a third triangle, are similar to each other ( transitivity of similarity of triangles).
* Corresponding altitudes of similar triangles have the same ratio as the corresponding sides.
* Two right triangles are similar if the hypotenuse and one other side have lengths in the same ratio. There are several equivalent conditions in this case, such as the right triangles having an acute angle of the same measure, or having the lengths of the legs (sides) being in the same proportion.

Given a triangle and a line segment one can, with ruler and compass, find a point such that . The statement that the point satisfying this condition exists is Wallis's postulate and is logically equivalent to Euclid's parallel postulate. In hyperbolic geometry (where Wallis's postulate is false) similar triangles are congruent.

In the axiomatic treatment of Euclidean geometry given by George David Birkhoff (see Birkhoff's axioms) the SAS similarity criterion given above was used to replace both Euclid's parallel postulate and the SAS axiom which enabled the dramatic shortening of Hilbert's axioms.

Similar triangles provide the basis for many synthetic (without the use of coordinates) proofs in Euclidean geometry. Among the elementary results that can be proved this way are: the angle bisector theorem, the geometric mean theorem, Ceva's theorem, Menelaus's theorem and the Pythagorean theorem. Similar triangles also provide the foundations for right triangle trigonometry.

Other similar polygons

The concept of similarity extends to polygons with more than three sides. Given any two similar polygons, corresponding sides taken in the same sequence (even if clockwise for one polygon and counterclockwise for the other) are proportional and corresponding angles taken in the same sequence are equal in measure. However, proportionality of corresponding sides is not by itself sufficient to prove similarity for polygons beyond triangles (otherwise, for example, all rhombi would be similar). Likewise, equality of all angles in sequence is not sufficient to guarantee similarity (otherwise all rectangles would be similar). A sufficient condition for similarity of polygons is that corresponding sides and diagonals are proportional.

For given ''n'', all regular ''n''-gons are similar.

Similar curves

Several types of curves have the property that all examples of that type are similar to each other. These include:
*Lines (any two lines are even congruent)
*Line segments
*Circles
*Parabolas
*Hyperbolas of a specific eccentricityThe shape of an ellipse or hyperbola depends only on the ratio b/a
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*Ellipses of a specific eccentricity
* Catenaries
*Graphs of the logarithm function for different bases
*Graphs of the exponential function for different bases
*Logarithmic spirals are self-similar

In Euclidean space

A similarity (also called a similarity transformation or similitude) of a Euclidean space is a bijection from the space onto itself that multiplies all distances by the same positive real number , so that for any two points and we have

:$d(f(x),f(y)) = r\, d(x,y), \,$

where "" is the Euclidean distance from to . The scalar has many names in the literature including; the ''ratio of similarity'', the ''stretching factor'' and the ''similarity coefficient''. When = 1 a similarity is called an isometry (rigid transformation). Two sets are called similar if one is the image of the other under a similarity.

As a map , a similarity of ratio takes the form

:$f(x) = rAx + t,$

where is an orthogonal matrix and is a translation vector.

Similarities preserve planes, lines, perpendicularity, parallelism, midpoints, inequalities between distances and line segments. Similarities preserve angles but do not necessarily preserve orientation, ''direct similitudes'' preserve orientation and ''opposite similitudes'' change it.

The similarities of Euclidean space form a group under the operation of composition called the ''similarities group ''. The direct similitudes form a normal subgroup of and the Euclidean group of isometries also forms a normal subgroup. The similarities group is itself a subgroup of the affine group, so every similarity is an affine transformation.

One can view the Euclidean plane as the complex plane, that is, as a 2-dimensional space over the reals. The 2D similarity transformations can then be expressed in terms of complex arithmetic and are given by (direct similitudes) and (opposite similitudes), where and are complex numbers, . When , these similarities are isometries.

Area ratio and volume ratio

The ratio between the areas of similar figures is equal to the square of the ratio of corresponding lengths of those figures (for example, when the side of a square or the radius of a circle is multiplied by three, its area is multiplied by nine — i.e. by three squared). The altitudes of similar triangles are in the same ratio as corresponding sides. If a triangle has a side of length and an altitude drawn to that side of length then a similar triangle with corresponding side of length will have an altitude drawn to that side of length . The area of the first triangle is, , while the area of the similar triangle will be . Similar figures which can be decomposed into similar triangles will have areas related in the same way. The relationship holds for figures that are not rectifiable as well.

The ratio between the volumes of similar figures is equal to the cube of the ratio of corresponding lengths of those figures (for example, when the edge of a cube or the radius of a sphere is multiplied by three, its volume is multiplied by 27 — i.e. by three cubed).

Galileo's square–cube law concerns similar solids. If the ratio of similitude (ratio of corresponding sides) between the solids is , then the ratio of surface areas of the solids will be , while the ratio of volumes will be .

Similarity with a center

If a similarity has exactly one invariant point: a point that the similarity keeps unchanged, then this only point is called "center" of the similarity.

On the first image below the title, on the left, one or another similarity shrinks a  regular polygon into a  concentric one, the vertices of which are each on a side of the previous polygon. This rotational reduction is repeated, so the initial polygon is extended into an abyss of regular polygons. The center of the similarity is the common center of the successive polygons. A red segment joins a vertex of the initial polygon to its image under the similarity, followed by a red segment going to the following image of vertex, and so on to form a spiral under a homothety of negative $\text-k,\,$ which is a similarity of ±180° angle and a positive ratio $\textk.$

Below the title on the right, the second image shows a similarity decomposed into a rotation and a homothety. Similarity and rotation have the same angle of +135 degrees modulo 360 degrees. Similarity and homothety have the same ratio $\text\,\frac,$ multiplicative inverse of the $\,\text\sqrt\;$ ( square root of 2) of the inverse similarity. Point ''S'' is the common center of the three transformations: rotation, homothety and similarity. For example point ''W'' is the image of ''F'' under the rotation, and point ''T'' is the image of ''W'' under the homothety, more briefly ''T '' = (''W '') = ( ''F'' )) = ( ''F'' ) = ( ''F ''), by naming $R,\;H\textD\,$ the previous rotation, homothety and similarity, with $\textD\,\text\,$

This direct similarity that transforms triangle ''EFA'' into triangle ''ATB'' can be decomposed into a rotation and a homothety of same center ''S'' in several manners. For example, $D\,=\,R\,\circ\,H\,=\,H\,\circ\,R,$ the last decomposition being only represented on the image. To get $D$ we can also compose in any order a rotation $\text-\,45^$ angle and a homothety $\,\text\,\frac.$

With $\textM\,\text\,$ and $\textI\,\text\,$ if $M\,$ is the reflection with respect to line (''CW'' ), then $M\,\circ\,D\,=\,I\;$ is the indirect similarity that transforms segment  'BF'' �$\textD\,$ into segment  'CT''  but transforms point ''E'' into ''B'' and point ''A'' into ''A'' itself. Square ''ACBT'' is the image of ''ABEF'' under similarity $I\,\text\;1\,/\sqrt.\;$ Point ''A'' is the center of this similarity because any point ''K'' being invariant under it fulfills $AK\,=\,AK/\sqrt,\;$ only possible $\text\,AK\,=\,0,\;$ otherwise written $A\,=\,K.$

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