If two sides of a triangle are congruent, then the angles opposite these sides are also congruent

Given

Isosceles triangle ABC, AB = AC

Prove
 

 Proof

1: AB = AC  -  Given
2: Draw angle bisector AO from A to BC
3: -  The bisector of the vertex angle of an isosceles triangle separates the triangle into two congruent triangles

4: -  CPCTC (Corresponding parts of congruent triangles are congruent)