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Square and Circumcircle Problem

Source:

May 1, 2012
geometrycircumcircletrigonometrygeometry unsolved

Problem Statement

A; B; C; D are the vertices of a square, and P is a point on the arc CD of its circumcircle. Prove that ∣PA∣2−∣PB∣2=∣PB∣.∣PD∣−∣PA∣.∣PC∣ |PA|^2 - |PB|^2 = |PB|.|PD| -|PA|.|PC| Can anyone here find the solution? I'm not great with geometry, so i tried turning it into co-ordinate geometry equations, but sadly to no avail. Thanks in advance.
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