A property of a tangential quadrilateral

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SOLUTION TO PROBLEM 1834. OF THE MATHEMATICS MAGAZINE. OMRAN KOUBA. Abstract. Let ABCD be a quadrilateral that has an inscribed circle with ...
A PROPERTY OF A TANGENTIAL QUADRILATERAL SOLUTION TO PROBLEM 1834 OF THE MATHEMATICS MAGAZINE

OMRAN KOUBA Abstract. Let ABCD be a quadrilateral that has an inscribed circle with center I, and let ` be a line tangent to the incircle. Let A0 , B 0 , C 0 and D0 , respectively, be the projections of A, B, C and D onto `. Then the following identity holds: AI · CI AA0 · CC 0 = . 0 0 BB · DD BI · DI

Problem 1834. [1]. Proposed by Cosmin Pohoata, student, National College “Tudor Vianu,” Bucharest, Romania. Let ABCD be a quadrilateral that has an inscribed circle with center I, and let ` be a line tangent to the incircle. Let A0 , B 0 , C 0 and D0 , respectively, be the projections of A, B, C and D onto `. Prove that AI · CI AA0 · CC 0 = . 0 0 BB · DD BI · DI Solution [2]: In our solution we will identify the plane with the complex numbers. The complex number representing a point denoted by a capital letter will be denoted by the corresponding small letter. Let us start by proving the following lemma : Lemma. Let S = {z ∈ C : |z| = 1} be the unit circle in the complex plane. (a) Consider two non-diametrically opposite points U and V from S, then the point of intersection of tangents to S from U and V is the point represented by the complex number 2uv/(u + v). (b) Consider a point Ω from S. For a given point Z in the plane, we define Z 0 as the projection of Z onto the tangent to S from Ω. Then ZZ 0 = |

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