If two sides of a triangle are congruent , then the angles opposite to these sides are congruent.
∠P≅∠Q∠P≅∠Q
Proof:
Let SS be the midpoint of PQ¯¯¯¯¯PQ¯ .
Join RR and SS .
Since SS is the midpoint of PQ¯¯¯¯¯PQ¯ , PS¯¯¯¯¯≅QS¯¯¯¯¯PS¯≅QS¯ .
By Reflexive Property ,
RS¯¯¯¯¯≅RS¯¯¯¯¯RS¯≅RS¯
It is given that PR¯¯¯¯¯≅RQ¯¯¯¯¯PR¯≅RQ¯
Therefore, by SSS ,
ΔPRS≅ΔQRSΔPRS≅ΔQRS
Since corresponding parts of congruent triangles are congruent,
∠P≅∠Q∠P≅∠Q
The converse of the Isosceles Triangle Theorem is also true.
If two angles of a triangle are congruent, then the sides opposite those angles are congruent.
If ∠A≅∠B∠A≅∠B , then AC¯¯¯¯¯≅BC¯¯¯¯¯AC¯≅BC¯ .