∠ADO = ∠APB = 90°
(angle subtended by diameter is always 90°)
⇒ OD\\PB

AO = OB (Radius of bigger circle)
(If in a triangle, the line drawn from midpoint of one side, is parallel to another side, then the line will bisect the third side)
Therefore AD = DP
(AB’s midpoint is ‘O’ and OD\\PB)