Answer:
See explanation
Step-by-step explanation:
If
then triangle PXY is isosceles triangle. Angles adjacent to the base XY of an isosceles triangle PXY are congruent, so

and

Angles 1 and 3 are supplementary, so

Angles 2 and 4 are supplementary, so

By substitution property,

Hence,

Consider triangles APX and BPY. In these triangles:
- given;
- given;
- proven,
so
by ASA postulate.
Congruent triangles have congruent corresponding sides, then

Therefore, triangle APB is isosceles triangle (by definition).