Maths Olympiad Prep

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Combinatorics Difficulty 5.9 AIME, harder Prove it Thailand

Let P1,P2,,P2556P_1, P_2, \dots, P_{2556} be distinct interior points of a regular hexagon ABCDEFABCDEF with side length 11. Assume that no three points of the set
S={A,B,C,D,E,F,P1,P2,,P2556} S = \{A, B, C, D, E, F, P_1, P_2, \dots, P_{2556}\}
are collinear. Show that there is a triangle with area less than 11700\frac{1}{1700} all of whose vertices belong to SS.

Solution

Draw line segments from P1P_1 to the points A,B,C,D,E,FA, B, C, D, E, F, we get six smaller triangles. Since any three points in SS are not collinear, P2P_2 is inside one of the six triangles. Draw line segments joining P2P_2 with the vertices of this triangle to divide it into three triangles. The total number of triangles is increased by 22. Do the same for the points P3,P4,,P2556P_3, P_4, \dots, P_{2556} to subdivide the hexagon into non-overlapping triangles, each time the total number of triangles is increased by 22. Thus, after the construction, the total number of triangles is 6+(2555×2)=51166 + (2555 \times 2) = 5116. If the area of each of these triangles is 11700\ge \frac{1}{1700}, then the area of the hexagon is 5116×11700>3>332=area of the regular hexagon ABCDEF\ge 5116 \times \frac{1}{1700} > 3 > \frac{3\sqrt{3}}{2} = \text{area of the regular hexagon } ABCDEF, which is a contradiction.

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