Because ABCD is rectangular.
So angle BAD= angle QBC=90 degrees.
Because PQ is perpendicular to PC
So CPQ angle =90 degrees.
So CPQ angle +QBC angle = 180 degrees.
So q, b, c, p four-point * * * circle.
So Angel ·QBD = Angel ·QCP
Because the angle is not good = angle CPQ=90 degrees.
So the right triangle BAD is similar to the right triangle CPQ (AA).
So: AD/PQ=AB/PC
So: PQ/PC=AD/AB