If a triangle has two angles equal to one another then the sides subtending the equal angles will also be equal to one another. Let $ABC$ be a triangle having the angle $ABC$ equal to the angle $ACB$. I say that side $AB$ is also equal to side $AC$.  <prf> Thus, $AB$ is not unequal to $AC$. Thus, (it is) equal. Thus, if a triangle has two angles equal to one another then the sides subtending the equal angles will also be equal to one another.