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ISL 2010 G7

    Three circular arcs γ1,γ2\gamma_{1}, \gamma_{2}, and γ3\gamma_{3} connect the points AA and CC. These arcs lie in the same half-plane defined by line ACA C in such a way that arc γ2\gamma_{2} lies between the arcs γ1\gamma_{1} and γ3\gamma_{3}. Point BB lies on the segment ACA C. Let h1,h2h_{1}, h_{2}, and h3h_{3} be three rays starting at BB, lying in the same half-plane, h2h_{2} being between h1h_{1} and h3h_{3}. For i,j=1,2,3i, j=1,2,3, denote by VijV_{i j} the point of intersection of hih_{i} and γj\gamma_{j} (see the Figure below). Denote by VijVkj^VkℓViℓ^\widehat{V_{i j} V_{k j}} \widehat{V_{k \ell} V_{i \ell}} the curved quadrilateral, whose sides are the segments VijViℓ,VkjVkℓV_{i j} V_{i \ell}, V_{k j} V_{k \ell} and arcs⁡VijVkj\operatorname{arcs} V_{i j} V_{k j} and ViℓVkℓV_{i \ell} V_{k \ell}. We say that this quadrilateral is circumscribed if there exists a circle touching these two segments and two arcs. Prove that if the curved quadrilaterals V11V2122V12,V12V2223V13,V21V31V32V22\sqrt{V_{11} V_{21}} \sqrt{22} V_{12}, \sqrt{V_{12} V_{22}} \sqrt{23} V_{13}, \sqrt{V_{21} V_{31}} \sqrt{V_{32} V_{22}} are circumscribed, then the curved quadrilateral V22V3233V23\sqrt{V_{22} V_{32}} \sqrt{33} V_{23} is circumscribed, too. Fig. 1 (Hungary)

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