Let and be two non-intersecting circles. Suppose the following three conditions hold: - The length of a common internal tangent of and is equal to 19 . - The length of a common external tangent of and is equal to 37 . - If two points and are selected on and , respectively, uniformly at random, then the expected value of is 2023 . Compute the distance between the centers of and .
Solution
The key claim is that . To prove this claim, choose an arbitrary point on . Let be the radii of respectively, and be the centers of respectively. Thus, by the law of cosines, , where . Since the average value of is 0 , the average value of is . Now suppose is an arbitrary point on . By the law of cosines, , where . Thus, the expected value of is the expected value of which becomes . This proves the key claim. Thus, we have . The lengths of the internal and the external tangents give us , and . Thus, Thus, .
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