In convex quadrilateral were selected points such that and , are on , - on . is inscribed. Prove that is inscribed too.
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Also problem 10.3 of 3rd (Regional) Round of Russian MO
Solution
1. Given that is cyclic, we know that and .
2. Since and , triangles and are isosceles.
3. Because is cyclic, we have . This follows from the fact that and are both subtended by the same arc in their respective circles.
4. Since and are isosceles, we have and .
5. Therefore, . This implies that quadrilateral is cyclic because opposite angles of a cyclic quadrilateral are supplementary.
6. Similarly, we can show that is cyclic. Since is cyclic, .
7. Let . We need to show that .
8. Since is cyclic, .
9. Since , we have .
10. Similarly, we can show that . Therefore, is cyclic.
Conclusion:
Since we have shown that is cyclic, the proof is complete.
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