Maths Olympiad Prep

Track / Stage 7 / 35 of 300 #1435 of 1964

Problem 1435

National olympiad second round; IMO P1/P4
Geometry Difficulty 7.1 Prove it

Given 2n2n points and 3n3n lines on the plane. Prove that there is a point PP on the plane such that the sum of the distances of PP to the 3n3n lines is less than the sum of the distances of PP to the 2n2n points.

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. **Consider an arbitrary point Q Q on the plane and a circle Ω \Omega centered at Q Q with radius R R .

2. Calculate the average distance from a point on Ω \Omega to any of the 3n 3n 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 R2π R \cdot \frac{2}{\pi} . This is derived from the integral of the absolute value of the sine function over one period:
02πsinxdx=4 \int_{0}^{2\pi} |\sin x| \, dx = 4
Dividing by the period 2π 2\pi , we get the average value:
42π=2π \frac{4}{2\pi} = \frac{2}{\pi}
- Therefore, the average distance from a point on Ω \Omega to any of the 3n 3n lines is approximately:
3nR2π=6nRπ 3n \cdot R \cdot \frac{2}{\pi} = \frac{6nR}{\pi}

3. **Calculate the average distance from a point on Ω \Omega to any of the 2n 2n points.**

- The average distance from a point on the circle to any of the 2n 2n points is approximately R R , since the distance from a point on the circle to a fixed point inside the circle is roughly the radius R R for large R R .
- Therefore, the average distance from a point on Ω \Omega to any of the 2n 2n points is approximately:
2nR=2nR 2n \cdot R = 2nR

4. Compare the two sums of distances.

- The sum of the distances from a point on Ω \Omega to the 3n 3n lines is:
6nRπ \frac{6nR}{\pi}
- The sum of the distances from a point on Ω \Omega to the 2n 2n points is:
2nR 2nR

5. **For sufficiently large R R , 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:
6nRπ<2nR \frac{6nR}{\pi} < 2nR
- Dividing both sides by nR nR (which is positive for n,R>0 n, R > 0 ):
6π<2 \frac{6}{\pi} < 2
- Since π3.14159 \pi \approx 3.14159 , we have:
6π63.141591.909<2 \frac{6}{\pi} \approx \frac{6}{3.14159} \approx 1.909 < 2
- This inequality holds true.

6. Conclusion:

- Therefore, for sufficiently large R R , the sum of the distances from a point on Ω \Omega to the 3n 3n lines is less than the sum of the distances from a point on Ω \Omega to the 2n 2n points.

\blacksquare

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