Olympiad Maths Prep

Track / Stage 6 / 204 of 400 #1204 of 2000

Problem 1204

National olympiad, first round
Combinatorics Difficulty 6.3 Find the answer

Consider a set of 10011001 points in the plane, no three collinear. Compute the minimum number of segments that must be drawn so that among any four points, we can find a triangle.

[i]Proposed by Ahaan S. Rungta / Amir Hossein[/i]

Official solution

To solve this problem, we need to ensure that among any four points chosen from the set of 1001 points, we can always find a triangle. This means that we need to avoid the situation where all four points are connected in such a way that they form a quadrilateral without any diagonals.

1. Understanding the Problem:
- We have 1001 points in the plane, and no three of them are collinear.
- We need to draw segments such that among any four points, there is always a triangle.

2. Key Insight:
- If we connect every pair of points, we will certainly have a triangle among any four points because every set of four points will have six segments connecting them, which guarantees at least one triangle.

3. Calculating the Total Number of Segments:
- The total number of segments that can be drawn between 1001 points is given by the combination formula (10012) \binom{1001}{2} , which represents the number of ways to choose 2 points out of 1001 to form a segment.
- This is calculated as:
(10012)=100110002=500500 \binom{1001}{2} = \frac{1001 \cdot 1000}{2} = 500500

4. Ensuring the Minimum Number of Segments:
- To ensure that among any four points there is always a triangle, we need to consider the minimum number of segments required.
- If we do not connect every pair of points, we risk having a set of four points that form a quadrilateral without any diagonals, which would not satisfy the condition.
- Therefore, the minimum number of segments required is indeed the total number of segments possible, which is 500500 500500 .

Conclusion:
500500 \boxed{500500}

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