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Difficulty: 4/102023 IOQM 2023 (Q18)

Let P be convex polygon with 50 vertices. A set F of diagonals of P is said to be minimally friendly if any diagonal in F intersects at most one other diagonal in F at point interior to P. Find the largest possible number of elements in minimally friendly set. F.

Guide / Hint

Hint 1: Start by analyzing the initial conditions and setting up the basic equations. Total number of non-intersecting diagonals.

Hint 2: Look for algebraic properties, symmetry, or geometric theorems to simplify. A1A3, A1A4, A1A5,.., A1A4, → 47.

Hint 3: Proceed with the final algebraic steps to solve the system. Total number of intersecting diagonals at only one point to the non-intersecting diagonals.

Solution

Step 1: Total number of non-intersecting diagonals

Step 2: A1A3, A1A4, A1A5,.., A1A4, → 47

Step 3: Total number of intersecting diagonals at only one point to the non-intersecting diagonals

Step 4: A2A4, A4A6, A6A8, …., A48A50 → 24

Step 5: Total = 47 + 24 = 71

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    2023 IOQM 2023 Q18 - Olympiad Math Olympiad Question | Leminno