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Explanation

The applet below provides an illustration to a problem from an outstanding collection by T. Andreescu and R. Gelca:

Let ABCD be a convex quadrilateral such that the diagonals AC and BD are perpendicular, and let P be their intersection. Prove that the reflections of P with respect to AB, BC, CD, and DA are concyclic.

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The quadrilateral in question is a dilation with coefficient 2 of the quadrilateral formed by projections of P on the sides of quadrilateral ABCD. It suffices to prove that the latter is cyclic. Let X, Y, Z, W be the feet of perpendiculars from P to the sides AB, BC, CD, DA. The quadrilaterals AXPW, BYPX, CZPY, DWPZ are cyclic as having a pair of opposite right angles. From this we obtain the following identities:

∠WAP = ∠WXP,
∠PXY = ∠PBY,
∠YZP = ∠YCP,
∠PZW = ∠PDW.

In triangles APD and BPC we have

 ∠WXY + ∠WZY = ∠WXP + ∠PXY + ∠YZP + ∠PZW = ∠WAP + ∠PDW + ∠PBY + ∠YCP = 90° = 90° = 180°,

which indeed shows that quadrilateral XYZW is cyclic.

### References

1. T. Andreescu, R. Gelca, Mathematical Olympiad Challenges, Birkhäuser, 2004, 5th printing, 1.2.5 (p. 9)

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