If
, then
is isosceles, and its base angles are congruent. So, angles
and
are congruent.
If
, then
is isosceles, and its base angles are congruent. So, angles
and
are congruent.
In the third case,
is isosceles, and the side adjacent to the congruent angles are congruent, so

Similarly, in this case,
is isosceles, and the side adjacent to the congruent angles are congruent, so
