Question 41
The bisector of the internal angle at the vertex $B$ of a triangle $ABC$ intersects the circumcircle of the triangle $ABC$ at a point $P$ ($P \neq B$). The line through $P$ perpendicular to the line $AC$ intersects the circumcircle of the triangle $ABC$ at a point $P'$ ($P' \neq P$). Prove that the quadrilateral $APCP'$ is a square if and only if $\angle ABC = 90^\circ$.