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Geometry Difficulty 6.3 National olympiad Find the answer

Find the minimum positive integer n3n\ge 3, such that there exist nn points A1,A2,,AnA_1,A_2,\cdots, A_n satisfying no three points are collinear and for any 1in1\le i\le n, there exist 1jn(ji)1\le j \le n (j\neq i), segment AjAj+1A_jA_{j+1} pass through the midpoint of segment AiAi+1A_iA_{i+1}, where An+1=A1A_{n+1}=A_1

A number or a short expression. Spacing and $ signs are ignored.

Solution

To find the minimum positive integer n3 n \geq 3 such that there exist n n points A1,A2,,An A_1, A_2, \ldots, A_n satisfying no three points are collinear and for any 1in 1 \leq i \leq n , there exists 1jn 1 \leq j \leq n (with ji j \neq i ), such that the segment AjAj+1 A_jA_{j+1} passes through the midpoint of segment AiAi+1 A_iA_{i+1} , where An+1=A1 A_{n+1} = A_1 , we proceed as follows:

First, it is necessary to verify that n=3 n = 3 and n=4 n = 4 do not satisfy the given conditions. Through geometric construction and analysis, it can be shown that no such configurations exist for these values of n n .

Next, consider n=5 n = 5 . We analyze two cases:
1. Case 1: There are no parallelograms formed by any four of the points Ai A_i . By detailed geometric analysis and coordinate bashing, it can be shown that no such five points exist.
2. Case 2: Assume A1A4A2A3 A_1A_4A_2A_3 forms a parallelogram. By considering the reflection of points and ensuring no three points are collinear, it leads to a contradiction, proving that n=5 n = 5 is also not possible.

Finally, for n=6 n = 6 , a construction exists that satisfies all the given conditions. Therefore, the minimum positive integer n n for which the conditions hold is n=6 n = 6 .

The answer is: \boxed{6}.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.