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Geometry Difficulty 6.6 National olympiad Prove it China

Find the minimal integer n3n \ge 3 such that there exist nn points A1,A2,,AnA_1, A_2, \dots, A_n on a plane with no three being colinear, such that for every 1in1 \le i \le n, the midpoint of the segment AiAi+1A_iA_{i+1} is contained in the segment AjAj+1A_jA_{j+1} for some jij \ne i. Here, An+1=A1A_{n+1} = A_1.

Solution

Solution: First, we prove that n=6n = 6 satisfies the condition. Let P1,P2,,P6P_1, P_2, \dots, P_6 be the vertices of a regular hexagon in consecutive order. Let A1=P1,A2=P3,A3=P5,A4=P2,A5=P6,A6=P4A_1 = P_1, A_2 = P_3, A_3 = P_5, A_4 = P_2, A_5 = P_6, A_6 = P_4. Then, these 6 points satisfy the conditions. See the figure below.
Figure 1

We now prove that n5n \le 5 does not satisfy the given conditions. Clearly, n=3n = 3 does not satisfy the condition because for any 1in,1jn1 \le i \le n, 1 \le j \le n in the statement, we require ji1,0,1(modn)j - i \ne -1, 0, 1 \pmod{n}.

When n=4n = 4, if A1,A2,A3,A4A_1, A_2, A_3, A_4 satisfy the conditions, then A1A2A_1A_2 and A3A4A_3A_4 must bisect each other, and A2A3A_2A_3 and A4A1A_4A_1 must bisect each other, which is impossible.

Therefore, we only need to consider the case when n=5n = 5. Suppose A1,A2,A3,A4,A5A_1, A_2, A_3, A_4, A_5 satisfy the conditions. For any 1i51 \le i \le 5, we require ji+2,i+3(mod5)j \equiv i + 2, i + 3 \pmod{5} in the statement. We consider two cases.

Case 1: Two of the line segments AiAi+1A_iA_{i+1}, i=1,2,3,4,5i = 1, 2, 3, 4, 5 bisect each other. By symmetry, we assume that A1A2A_1A_2 and A3A4A_3A_4 bisect each other. Note that applying an affine transformation to the plane does not affect the conclusion, we can assume that A1A2A_1A_2 and A3A4A_3A_4 are perpendicular bisectors and have equal length. Let OO be their intersection point, and let us establish a Cartesian coordinate system with OO as the origin. Let A1(0,2),A2(0,2),A3(2,0),A4(2,0)A_1(0, -2), A_2(0, 2), A_3(2, 0), A_4(-2, 0). Then the midpoint MM of A2A3A_2A_3 must lie on either A4A5A_4A_5 or A1A5A_1A_5. Without loss of generality, we assume that MM lies on A1A5A_1A_5. Note that the slope of A1MA_1M is 3, so we can assume that the coordinates of A5A_5 are (1+x,1+3x)(1 + x, 1 + 3x), where x>0x > 0. See the figure below.
Figure 2

Now consider the midpoint NN of A4A5A_4A_5. If it lies inside the segment A1A2A_1A_2, then 1+x+(2)=01+x+(-2) = 0 implies x=1x = 1. However, in this case A4,A2,A5A_4, A_2, A_5 are collinear, a contradiction. If NN lies inside the segment A2A3A_2A_3, then
1+x+(2)2+1+3x2=2, \frac{1+x+(-2)}{2} + \frac{1+3x}{2} = 2,
also yields x=1x = 1, which is a contradiction.

Case 2: None of the segments AiAi+1A_iA_{i+1}, i=1,2,3,4,5i = 1, 2, 3, 4, 5 bisect each other. Without loss of generality, assume that the midpoint B1B_1 of A1A2A_1A_2 lies on A3A4A_3A_4. Then the midpoint B3B_3 of A3A4A_3A_4 lies on A5A1A_5A_1, the midpoint B5B_5 of A5A1A_5A_1 lies on A2A3A_2A_3, the midpoint B2B_2 of A2A3A_2A_3 lies on A4A5A_4A_5, and the midpoint B4B_4 of A4A5A_4A_5 lies on A1A2A_1A_2. This implies that connecting A1,A2,A3,A4,A5A_1, A_2, A_3, A_4, A_5 in sequence yields a five-pointed star.
Figure 3

By the Law of Sines,
1=i=15sinAiBiBi+2sinAiBi+2Bi=i=15AiBi+2AiBi=i=15Ai+1Bi+3AiBi<i=15Ai+1BiAiBi=1, 1 = \prod_{i=1}^{5} \frac{\sin \angle A_i B_i B_{i+2}}{\sin \angle A_i B_{i+2} B_i} = \prod_{i=1}^{5} \frac{A_i B_{i+2}}{A_i B_i} = \prod_{i=1}^{5} \frac{A_{i+1} B_{i+3}}{A_i B_i} < \prod_{i=1}^{5} \frac{A_{i+1} B_i}{A_i B_i} = 1,
where the subscripts are understood modulo 5. This is a contradiction.

Therefore, the minimum nn we seek is 6.

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