[Math] Prove that a triangle that has two congruent angles is isosceles

Question

I'm having some trouble with the following problem:

Prove that a triangle that has two congruent angles is isosceles

I tried to prove this by separating it into two triangles and use the ASA or the SAS postulate. However, I am stuck. I need some help. Thank you!

Answer 1

We can do it without drawing any line.

Let our triangle be $ABC$, with $\angle B=\angle C$.

By ASA, $\triangle ABC$ and $\triangle ACB$ are congruent.