The figure below shows a cyclic quadrilateral ABCD with perpendicular diagonals at P. Lines H1PE3, H2PE4, H3PE1 , and H4PE2 are perpendicular to AB, BC, CD, and AD, respectively. Prove that the quadrilateral E1E2E3E4 is a rectangle.
	     
 

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						Cyclic Quadrilateral with Perpendicular Diagonals
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