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Combinatorics Difficulty 7.2 National olympiad, round 2 Prove it Argentina

Consider 20182018 points placed on the vertices of regular hexagons as shown in the picture:
Figure 1
A bee and a beetle play the following game: initially, the bee chooses one of the 20182018 points and paints it yellow; then, the beetle chooses one of the 20172017 points that have not been painted and paints it green. They continue playing in this way: the bee chooses one unpainted point and paints it yellow and, then, the beetle chooses one unpainted point and paints it green. When all points have been painted, if there is an equilateral triangle with its three vertices painted the same color, the bee wins. Otherwise, the beetle wins. Determine which of them has a winning strategy.

Solution

First, we analyze in which cases three of the considered points form an equilateral triangle.
Let **ABCABC** be an equilateral triangle with vertices among the points.

*Case 1:* One vertex AA is at a point in the lower part of a hexagon, as in the picture (the case when it is in the upper part of a hexagon is analogous).
Figure 2
Assume the vertex BB is in the half-plane to the left of the line **AYAY** (the other case is similar). If B=YB = Y, then C=XC = X, and if B=WB = W, then CC cannot be a vertex of one of the hexagons. So, we may now assume that BB and CC are both in the region determined by the half-lines AX\vec{AX} and AZ\vec{AZ}. If BXB \neq X, since XA^Z=60X \hat{A} Z = 60^\circ, we have that BA^C<60B \hat{A} C < 60^\circ, a contradiction. Then B=XB = X and C=ZC = Z.

*Case 2:* All vertices are points on the vertical sides of the hexagons. Assume AA is as in the following picture.
Figure 3
Assume CC is in the half-plane to the right of the line **ATAT** (the other case is similar). If C=TC = T or C=UC = U, the third vertex BB of the equilateral triangle does not lie in the vertex of an hexagon. Then, both CC and BB would be in the region determined by the half-lines AU\vec{AU} and AV\vec{AV}, but then, BA^C<60B \hat{A} C < 60^\circ. Contradiction.

Summarizing, the equilateral triangles with vertices in the 20182018 points are those marked in the figure below:
Figure 4

Now, consider the following numbering of the 20182018 points
Figure 5
and the 10091009 pairs:
{A1,A2},{A3,A4},,{A1007,A1008},{B1,B2},{B3,B4},,{B1007,B1008},{X,Y}. \{A_1, A_2\}, \{A_3, A_4\}, \dots, \{A_{1007}, A_{1008}\}, \{B_1, B_2\}, \{B_3, B_4\}, \dots, \{B_{1007}, B_{1008}\}, \{X, Y\}.
The beetle has a winning strategy. It wins the game by playing as follows: every time the bee chooses and paints a point in one of the above pairs, the beetle chooses and paints the other point in the same pair. Note that every equilateral triangle with vertices in the 20182018 points has two vertices in the same pair; so, if its three vertices were painted the same color, there should be one pair with the two points painted the same color, which cannot happen if the beetle plays according to the strategy.

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Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.