In the convex quadrilateral（凸四邊形） ABCD, the diagonals AC and BD
are perpendicular and the opposite sides AB and DC are not parallel.
Suppose that the point P,where the perpendicular bisectors（中垂線）
of AB and DC meet, is inside ABCD.Prove that ABCD is a cyclic
quadrilateral（圓內接四邊形） if and only if the triangles ABP and CDP
have equal areas.
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