2020 AIME II Problems/Problem 7
Two congruent right circular cones each with base radius and height have the axes of symmetry that intersect at right angles at a point in the interior of the cones a distance from the base of each cone. A sphere with radius lies withing both cones. The maximum possible value of is , where and are relatively prime positive integers. Find .
Take the cross-section of the plane of symmetry formed by the two cones. Let the point where the bases intersect be the origin, , and the bases form the positive and axes. Then label the vertices of the region enclosed by the two triangles as in a clockwise manner. We want to find the radius of the inscribed circle of . By symmetry, the center of this circle must be . can be represented as Using the point-line distance formula, ~mn28407
Solution 2 (Official MAA)
Consider the cross section of the cones and sphere by a plane that contains the two axes of symmetry of the cones as shown below. The sphere with maximum radius will be tangent to the sides of each of the cones. The center of that sphere must be on the axis of symmetry of each of the cones and thus must be at the intersection of their axes of symmetry. Let be the point in the cross section where the bases of the cones meet, and let be the center of the sphere. Let the axis of symmetry of one of the cones extend from its vertex, , to the center of its base, . Let the sphere be tangent to at . The right triangles and are similar, implying that the radius of the sphere isThe requested sum is .
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