Maths Olympiad Prep

Track / Stage 7 / 187 of 300 #1587 of 1964

Problem 1587

National olympiad second round; IMO P1/P4
Geometry Difficulty 7.3 Prove it

Let A1,A2,...,A2002 A_1,A_2,...,A_{2002} be arbitrary points in the plane. Prove that for every circle of radius 1 1 and for every rectangle inscribed in this circle, there exist 33 vertices M,N,P M,N,P of the rectangle such that
MA1+MA2++MA2002+ MA_1 + MA_2 + \cdots + MA_{2002} + NA1+NA2++NA2002+NA_1 + NA_2 + \cdots + NA_{2002} + PA1+PA2++PA20026006PA_1 + PA_2 + \cdots + PA_{2002}\ge 6006.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

1. Define the vertices of the rectangle and the points in the plane:
Let W,X,Y,Z W, X, Y, Z be the vertices of the rectangle inscribed in the circle of radius 1. Let A1,A2,,A2002 A_1, A_2, \ldots, A_{2002} be arbitrary points in the plane.

2. Sum of distances from a point to the vertices of the rectangle:
Consider a point A A in the plane. The sum of the distances from A A to the vertices of the rectangle is given by:
WA+XA+YA+ZA WA + XA + YA + ZA

3. Use the properties of the rectangle and the circle:
Since the rectangle is inscribed in the circle, the diagonals of the rectangle are equal to the diameter of the circle, which is 2 (since the radius is 1). Let d1 d_1 and d2 d_2 be the lengths of the diagonals of the rectangle. Then:
d1=d2=2 d_1 = d_2 = 2

4. Apply the triangle inequality:
For any point A A in the plane, the sum of the distances from A A to the vertices of the rectangle can be split into pairs of opposite vertices:
WA+YAWYandXA+ZAXZ WA + YA \geq WY \quad \text{and} \quad XA + ZA \geq XZ
Since WY=XZ=2 WY = XZ = 2 , we have:
WA+YA+XA+ZA4 WA + YA + XA + ZA \geq 4

5. **Sum over all points A1,A2,,A2002 A_1, A_2, \ldots, A_{2002} :**
Summing the above inequality over all points A1,A2,,A2002 A_1, A_2, \ldots, A_{2002} , we get:
k=12002(WAk+YAk+XAk+ZAk)4×2002=8008 \sum_{k=1}^{2002} (WA_k + YA_k + XA_k + ZA_k) \geq 4 \times 2002 = 8008

6. Select the largest three terms:
Since the sum of the distances from all four vertices to all points is at least 8008, the sum of the distances from the three vertices with the largest sums will be at least:
34×8008=6006 \frac{3}{4} \times 8008 = 6006

Therefore, there exist three vertices M,N,P M, N, P of the rectangle such that:
k=12002MAk+k=12002NAk+k=12002PAk6006 \sum_{k=1}^{2002} MA_k + \sum_{k=1}^{2002} NA_k + \sum_{k=1}^{2002} PA_k \geq 6006

\blacksquare

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.