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Difficulty: 5/102023 IOQM 2023 (Q25)

Find the least positive integer such that there are at least 1000 unordered pairs of diagonals in regular polygon with vertices that intersect at right angle in the interior of the polygon.

Guide / Hint

Hint 1: Start by analyzing the initial conditions and setting up the basic equations. Let = 4k.

Hint 2: Look for algebraic properties, symmetry, or geometric theorems to simplify. (1 + 3 + 5 + \dots(2k − 1) + \dots3 + 1)  k.

Hint 3: Proceed with the final algebraic steps to solve the system. = (k 2 + (k − 1)2 )k  1000.

Solution

Step 1: Let = 4k

Step 2: (1 + 3 + 5 + \dots(2k − 1) + \dots3 + 1)  k

Step 3: = (k 2 + (k − 1)2 )k  1000

Step 4: => (2k 2 − 2k + 1)  1000

Step 5: =>  9 as in N

Step 6: =>  36

Step 7: (1 + 3 + \dots2k − 1) \times 2 \times (2k + 1)

Step 8: => (2k + 1)  2k 2  1000

Step 9: => 2 (2k + 1)  500

Step 10: =>  30

Step 11: => min(36, 30) = 30

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    2023 IOQM 2023 Q25 - Olympiad Math Olympiad Question | Leminno