Rule

Angle-Angle-Side Congruence Theorem

If two angles and a non-included side of a triangle are congruent to two angles and the corresponding non-included side of another triangle, then the triangles are congruent.

Based on the diagram above, the theorem can be written as follows.

Proof

This proof will be developed based on the given diagram, but it is valid for any pair of triangles.

Triangles ABC and DEF
The primary purpose of the proof is finding a rigid motion or sequence of rigid motions that maps one triangle onto the other. This can be done in several ways. One of the ways will be shown here.
1
Translate So That Two Corresponding Vertices Match
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Translate so that is mapped onto If this translation maps onto the proof is complete.
DEF is translated
Since the image of the translation does not match at least one more transformation is needed.
2
Rotate So That Two Corresponding Sides Match
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Rotate clockwise about so that a pair of corresponding sides match. If the image of this transformation is the proof is complete. Note that this rotation maps onto Therefore, the rotation maps onto
Translation of CD'E'
As before, the image does not match Therefore, a third rigid motion is required.
3
Reflect So That Two More Corresponding Sides Match
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It is given that two angles of are congruent to two angles of Hence, by the Third Angle Theorem, is congruent to

Triangles ABC and CBD'' with a common side CB
Reflect across Because reflections preserve angles, and are mapped onto and respectively. Then, the point of intersection of the original segments is mapped onto the point of intersection of the image segments
Reflection of CBD'' across BC
This time the image matches
Consequently, after a sequence of rigid motions, can be mapped onto This means that and are congruent triangles. The proof is complete.
Exercises