Maths Olympiad Prep

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Geometry Difficulty 5.2 AIME, harder Find the answer

Let YY be as in problem 14. Find the maximum ZZ such that three circles of radius Z\sqrt{Z} can simultaneously fit inside an equilateral triangle of area YY without overlapping each other.

A number or a short expression. Spacing and $ signs are ignored.

Solution

We first find that, in problem 15, each of the circles of radius Z\sqrt{Z} is the incircle of a 30609030^{\circ}-60^{\circ}-90^{\circ} triangle formed by cutting the equilateral one in half. The equilateral triangle itself has sidelength 2Y34\frac{2 \sqrt{Y}}{\sqrt[4]{3}}, so the said inradius is Z=1+322122Y34\sqrt{Z}=\frac{1+\sqrt{3}-2}{2} \cdot \frac{1}{2} \cdot \frac{2 \sqrt{Y}}{\sqrt[4]{3}} so that Z=(1+3)243Y=42343Y=2336YZ=\frac{(-1+\sqrt{3})^{2}}{4 \sqrt{3}} Y=\frac{4-2 \sqrt{3}}{4 \sqrt{3}} Y=\frac{2 \sqrt{3}-3}{6} Y Now we guess that X=2X=2 and see that, miraculously, everything works: in the problem 14, say a crimson flower is placed first. Then there are 2 possibilities for CC1,4C_{-} C_{-{ }_{-1}}, 4 for CC,2C_{-} C_{--}, 2 for CCC_{-\ldots} C_{-}, and 2 for CCC_{\ldots_{-}} C, giving a total of 10. Of course, the first flower can be of any of the three hues, so Y=310=30Y=3 \cdot 10=30. We compute ZZ and check XX in a straightforward manner. If X>2X>2, then Y>30Y>30, and Z>10315Z>10 \sqrt{3}-15, with the result that X2X \leq 2, a contradiction. Assuming X<2X<2 results in a similar contradiction.

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