ABC is an isosceles triangle with AB = AC. Draw AP⊥BC to show that Angle B = Angle C.

In ΔABP and ΔACP,
∠APB = ∠APC [∵ Each 90°]
AB = AC [∵ Given]
AP = AP [∵ Common]
Hence, ΔABP ≅ ΔACP [∵ RHS Congruency Rule]
∠B = ∠C [∵ CPCT]