△ ABC is an isosceles triangle with AB ¯¯¯¯¯¯≅ AC¯¯¯¯¯¯.

​ AN¯¯¯¯¯ ⊥ BC¯¯¯¯¯
​Prove that Part A
The first step in proving ​ ≅ ΔABN ≅ ΔACN. Using the given and the fact that AN¯¯¯¯¯ ≅ AN¯¯¯¯¯, this can be proved using which theorem?