Videos and lessons to help High School students learn when given two figures, use the definition of similarity in terms of similarity transformations to decide if they are similar; explain using similarity transformations the meaning of similarity for triangles as the equality of all corresponding pairs of angles and the proportionality of all corresponding pairs of sides.

Common Core: HSG-SRT.A.2

Related Topics:

Common
Core (Geometry)

Common Core
for Mathematics

Use transformations to prove similarity.

Similarity Transformations

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Geometry2D - Similarity

we can define similarity of two geometrical objects on a plane as possibility to transform one into another using dilation (scaling), as defined above, optionally combined with congruent transformations of parallel shift (translation), rotation and symmetry (reflection).

Geometry2D - Similarity - Triangles

Recall that we have defined similarity as the characteristic of one geometrical object to be an image of another after transformation of scaling and, possibly, some congruent transformation (translation, rotation and symmetry relative to an axis). Applied to triangles, we see, first of all, that an image of a triangle after transformation of scaling is a triangle (since straight lines are transformed into straight lines). We also observe that similar triangles have corresponding angles congruent (since scaling (dilation) preserves angles) and corresponding sides proportional with the same coefficient of proportionality equal to a scaling factor (since transformation of scaling changes the lengths of all segments by the same scaling factor). All in all, scaling transforms a triangle into another triangle, similar to original (by definition of similarity), with correspondingly congruent angles and correspondingly proportional sides.

The property of triangles to have congruent angles and proportional sides is, actually, equivalent to their similarity. In fact, three much shorter statements are true, each one, obviously, necessary and, as we are going to prove, sufficient conditions for similarity of triangles.

Theorem 1. If two triangles have two pairs of angles correspondingly congruent to each other, then they are similar.

Theorem 2. If two triangles have one pairs of congruent angles and sides, forming these angles, are proportional, then they are similar.

Theorem 3. If three sides of one triangle are correspondingly proportional to three sides of another triangle, then these triangles are similar.

Geometry2D - Similarity - Circles

Theorem 1 Scaling of a circle results in a circle. The radii of these circles are related by a scaling factor.

Theorem 2 Any two circles are similar.

Theorem 3 For any two circles, positioned outside of each other, there is a transformation of scaling that transforms one into the other.

Similarity of Circles

How to prove all circles are similar.

Quadrilateral similarity by showing congruent angles.

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