
Triangle Similarity Test  Two sides and included angle (SAS)
Definition: Triangles are similar if two sides in one triangle are in the same proportion to the corresponding sides in the other,
and the
included angle
are equal.
This (SAS) is one of the three ways to test that two triangles are similar . For a list see
Similar Triangles.
Try this
Drag any orange dot at P,Q,R. The triangle LMN will change to remain similar to the left triangle PQR.
If two sides in one triangle are in the same proportion to the corresponding sides in the other,
and the
included angle
between them is the same
then the triangles are similar.
For example in the triangle above, the side PQ is exactly twice
as long as the corresponding side LM in the other triangle, and PR is twice LN.
So these two sides are in the same proportion, in this case 2:1 (two to one).
Also the
included angle angles (P and L) are equal in
measure, and so the triangles are similar.
Notice that the two sides and the angle are drawn in a magenta color to show they are the things being used to test for similarity.
What does this mean?
Since two corresponding pairs of sides are in the same proportion, and the
included angle
angles are equal,
we can be sure the triangles are similar.
Because the triangles are similar:
 The three angles at P,Q and R are equal
to the angles L,M and N respectively.
 The corresponding sides in each triangle will all be in the same proportion.
But don't forget
Similar triangles can be rotated and/or mirror images of each other (reflected).
(See Similar triangles.)
In the figure on the right, the two triangles are still similar, even though one is the mirror image of the other and rotated.
Related topics
Similar Polygons
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