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Geometry Difficulty 5.2 AIME, harder Find the answer

Let ω1\omega_{1} and ω2\omega_{2} be two non-intersecting circles. Suppose the following three conditions hold: - The length of a common internal tangent of ω1\omega_{1} and ω2\omega_{2} is equal to 19 . - The length of a common external tangent of ω1\omega_{1} and ω2\omega_{2} is equal to 37 . - If two points XX and YY are selected on ω1\omega_{1} and ω2\omega_{2}, respectively, uniformly at random, then the expected value of XY2X Y^{2} is 2023 . Compute the distance between the centers of ω1\omega_{1} and ω2\omega_{2}.

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

Solution

The key claim is that E[XY2]=d2+r12+r22\mathbb{E}\left[X Y^{2}\right]=d^{2}+r_{1}^{2}+r_{2}^{2}. To prove this claim, choose an arbitrary point BB on ω2\omega_{2}. Let r1,r2r_{1}, r_{2} be the radii of ω1,ω2\omega_{1}, \omega_{2} respectively, and O1,O2O_{1}, O_{2} be the centers of ω1,ω2\omega_{1}, \omega_{2} respectively. Thus, by the law of cosines, O1B=d2+r222r2dcos(θ)\overline{O_{1} B}=\sqrt{d^{2}+r_{2}^{2}-2 r_{2} d \cos (\theta)}, where θ=O1O2B\theta=\angle O_{1} O_{2} B. Since the average value of cos(θ)\cos (\theta) is 0 , the average value of O1B2{\overline{O_{1} B}}^{2} is d2+r22d^{2}+r_{2}^{2}. Now suppose AA is an arbitrary point on ω1\omega_{1}. By the law of cosines, AB2=O1B2+r122r1dcos(θ)\overline{A B}^{2}={\overline{O_{1} B}}^{2}+r_{1}^{2}-2 r_{1} d \cos (\theta), where θ=AO1B\theta=\angle A O_{1} B. Thus, the expected value of AB2\overline{A B}^{2} is the expected value of O1B2+r12{\overline{O_{1} B}}^{2}+r_{1}^{2} which becomes d2+r12+r22d^{2}+r_{1}^{2}+r_{2}^{2}. This proves the key claim. Thus, we have d2+r12+r22=2023d^{2}+r_{1}^{2}+r_{2}^{2}=2023. The lengths of the internal and the external tangents give us d2(r1+r2)2=361d^{2}-\left(r_{1}+r_{2}\right)^{2}=361, and d2(r1r2)2=1369d^{2}-\left(r_{1}-r_{2}\right)^{2}=1369. Thus, d2(r12+r22)=(d2(r1+r2)2)+(d2(r1r2)2)2=361+13692=865d^{2}-\left(r_{1}^{2}+r_{2}^{2}\right)=\frac{\left(d^{2}-\left(r_{1}+r_{2}\right)^{2}\right)+\left(d^{2}-\left(r_{1}-r_{2}\right)^{2}\right)}{2}=\frac{361+1369}{2}=865 Thus, d2=865+20232=1444d=38d^{2}=\frac{865+2023}{2}=1444 \Longrightarrow d=38.

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