Olympiad Maths Prep

Track / Stage 6 / 221 of 400 #1221 of 2000

Problem 1221

National olympiad, first round
Geometry Difficulty 6.3 Find the answer

Show that given any 9 points inside a square of side 1 we can always find 3 which form a triangle with area less than 18\frac 18.

[i]Bulgaria[/i]

Official solution

1. Divide the Square into Smaller Regions:
Consider a unit square with side length 1. Divide this square into 4 smaller squares, each with side length 12\frac{1}{2}. Each of these smaller squares has an area of (12)2=14\left(\frac{1}{2}\right)^2 = \frac{1}{4}.

2. Pigeonhole Principle:
By the pigeonhole principle, if we place 9 points inside the unit square, at least one of the smaller squares must contain at least 3 of these points. This is because there are only 4 smaller squares and 9 points, so at least one square must contain 94=3\left\lceil \frac{9}{4} \right\rceil = 3 points.

3. Area of Triangle in Smaller Square:
Consider one of these smaller squares with side length 12\frac{1}{2}. Any triangle formed by 3 points within this smaller square will have an area less than or equal to the area of the smaller square. The maximum area of a triangle within this smaller square is achieved when the triangle is right-angled and its legs are the sides of the smaller square. The area of such a triangle is:
Area=12×12×12=18 \text{Area} = \frac{1}{2} \times \frac{1}{2} \times \frac{1}{2} = \frac{1}{8}

4. Conclusion:
Therefore, any triangle formed by 3 points within this smaller square will have an area less than or equal to 18\frac{1}{8}. Since we have shown that at least one of the smaller squares must contain at least 3 of the 9 points, we can conclude that there always exists a triangle with an area less than or equal to 18\frac{1}{8}.

True \boxed{\text{True}}

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