Let be arbitrary points in the plane. Prove that for every circle of radius and for every rectangle inscribed in this circle, there exist vertices of the rectangle such that
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Problem 1587
Official solution
1. Define the vertices of the rectangle and the points in the plane:
Let be the vertices of the rectangle inscribed in the circle of radius 1. Let be arbitrary points in the plane.
2. Sum of distances from a point to the vertices of the rectangle:
Consider a point in the plane. The sum of the distances from to the vertices of the rectangle is given by:
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 and be the lengths of the diagonals of the rectangle. Then:
4. Apply the triangle inequality:
For any point in the plane, the sum of the distances from to the vertices of the rectangle can be split into pairs of opposite vertices:
Since , we have:
5. **Sum over all points :**
Summing the above inequality over all points , we get:
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:
Therefore, there exist three vertices of the rectangle such that: