Maths Olympiad Prep

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Geometry Difficulty 5.8 AIME, harder Prove it Thailand

In a square of side length 55, a figure is placed such that the distance between any two of its points is not equal to 0.0010.001, 0.002×30.002 \times \sqrt{3}, 0.0020.002. Prove that the area of this figure does not exceed 3.5753.575.

Solution

Let FF denote this figure. Consider the translations of FF under the following vectors in the plane: u1=0.001i\vec{u}_1 = 0.001\vec{i}, and
uk+1=Rπ/3(uk),k=1,2,3,4,5, \vec{u}_{k+1} = R_{\pi/3}(\vec{u}_k), \quad k = 1, 2, 3, 4, 5,
where Rπ/3R_{\pi/3} denotes the rotation by π/3\pi/3 counterclockwise. Now let F0=FF_0 = F and, for k=1,,6k = 1, \dots, 6,
FkF_k = the translation of FF in the direction uk\vec{u}_k.

We will show that FkFl=F_k \cap F_l = \emptyset for all k,l{0,,6},klk, l \in \{0, \dots, 6\}, k \neq l. Since the distance between any two points of FF is not equal to 0.0010.001, F0F_0 and FkF_k have no common points for k=1,,6k = 1, \dots, 6. By the same reasoning, FkF_k and Fk+1F_{k+1} have no common points for k=1,,6k = 1, \dots, 6 (F7=F1F_7 = F_1). Next since the distance between any two points in FF is not equal to 0.001×30.001 \times \sqrt{3}, it follows that FkF_k and Fk+2F_{k+2} have no common points. Finally, that the distance between any two points in FF is not equal to 0.0020.002 implies FkF_k and Fk+2F_{k+2} have no points in common. Thus FkFl=F_k \cap F_l = \emptyset for all klk \neq l as claimed.

The figures FkF_k are all lying in the square of side 5.0025.002, and they have pairwise empty intersection. Thus
7×area(F)=i=06area(Fi)5.0022; 7 \times \text{area}(F) = \sum_{i=0}^{6} \text{area}(F_i) \le 5.002^2;
therefore area(F)5.0022/7<3.575\text{area}(F) \le 5.002^2/7 < 3.575. \square

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