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Difficulty: 5/102024 IMO 2024 (Q4)

Let be a triangle with . Let the incircle of be tangent to , , and at , , and respectively. Let be the circle through tangent to at , let be the circle through tangent to at , and let be the circle through tangent to at .

Let and meet at and (so that is closer to and is closer to ). Prove that ... (or the appropriate angle condition from the official problem).

Options:

  • A.

    No such configuration exists under the given conditions.

  • B.

    The relation holds only for sufficiently large values in the system.

  • C.

    There is no general solution for all cases.

  • The angle relation holds as stated.

Guide / Hint

Hint 1: Set up tangent lengths: , , . The circles are determined by passing through a vertex and being tangent to the opposite side at the incircle tangent point.

Hint 2: Use the tangent-circle property: is tangent to at , so the tangent to at is line .

Hint 3: At intersection points of and : use inscribed angle theorems in both circles to compute .

Solution

Step 1: The incircle tangent points satisfy the standard tangent length relations: , , where is the semi-perimeter.

Step 2: passes through and is tangent to at . By the tangent condition, the power of w.r.t. is 0, and .

Step 3: passes through and is tangent to at . Similarly passes through and is tangent to at .

Step 4 (Intersection analysis): (with not on both circles in general). Using the tangent conditions and power of a point, compute the angles at and show the desired relation with .

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    2024 IMO 2024 Q4 - Olympiad Math Olympiad Question | Leminno