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Similar triangles
It is helpful if students are also familiar with the tests for congruence.
There are four similarity tests for triangles.

Angle Angle Angle (AAA)
If two angles of one triangle are respectively equal to two angles of another triangle, then the two triangles are similar.
It is sufficient to prove that only two pairs of angles are respectively equal to each other. Because the angle sum of a triangle is always 180°, the third pair of angles will automatically be equal. Restating this fact is not required when using the AAA test in a similarity proof. 
Side Angle Side (SAS)
If the ratio of the lengths of two sides of one triangle is equal to the ratio of the lengths of two sides of another triangle, and the included angles are equal, then the two triangles are similar. 
Side Side Side (SSS)
If each of the sides of one triangle can be matched up with each of the sides of another so that the ratios of matching sides are equal, then the two triangles are similar. 
Rightangle Hypotenuse Side (RHS)
If the ratio of the hypotenuse and one side of a rightangled triangle is equal to the ratio of the hypotenuse and one side of another rightangled triangle, then the two triangles are similar.
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