Given points and lines on the plane. Prove that there is a point on the plane such that the sum of the distances of to the lines is less than the sum of the distances of to the points.
Problem 1435
Official solution
1. **Consider an arbitrary point on the plane and a circle centered at with radius .
2. Calculate the average distance from a point on to any of the lines.**
- For a line in the plane, the distance from a point on the circle to the line can be approximated by considering the average distance over all points on the circle.
- The average distance from a point on the circle to a line is given by . This is derived from the integral of the absolute value of the sine function over one period:
Dividing by the period , we get the average value:
- Therefore, the average distance from a point on to any of the lines is approximately:
3. **Calculate the average distance from a point on to any of the points.**
- The average distance from a point on the circle to any of the points is approximately , since the distance from a point on the circle to a fixed point inside the circle is roughly the radius for large .
- Therefore, the average distance from a point on to any of the points is approximately:
4. Compare the two sums of distances.
- The sum of the distances from a point on to the lines is:
- The sum of the distances from a point on to the points is:
5. **For sufficiently large , show that the sum of the distances to the lines is less than the sum of the distances to the points.**
- We need to show:
- Dividing both sides by (which is positive for ):
- Since , we have:
- This inequality holds true.
6. Conclusion:
- Therefore, for sufficiently large , the sum of the distances from a point on to the lines is less than the sum of the distances from a point on to the points.