In △s, ABD and ACD,
AD=AD (Common)
AB=AC (Given, ABC is an isosceles triangle)
DB=DC (Given, DBC is an isosceles triangle)
Thus, △ABD≅△ACD (SSS postulate)
Hence, PB=PC (By cpct)
and, ∠APB=∠APC=x (By cpct)
Now, ∠APB+∠APC=180
x+x=180
x=90∘
Thus, AP is perpendicular bisector of BC