Maths Olympiad Prep

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Geometry Difficulty 5.5 AIME, harder Prove it Iran

Consider a spherical ball on a plane and a point marked on it. We want to roll the ball on a closed polygon of the plane to bring the mark to the top of the ball while the ball is on its first place. Note that the ball must not roll in its place (it means rolling without moving on the plane). Prove that this is possible.

Figure 1

Solution

Let rr be the radius of the ball and OO be its first position on the plane. First, we roll the ball (sphere) on its great circle that passes through the marked point and the top point of the ball, until the mark be on top of the ball. Let AA be the current position of the ball on the plane. It is trivial that OA<2πrOA < 2\pi r. So there exists a point BB on the plane so that AB=OB=2πrAB = OB = 2\pi r. Now we roll the ball from AA to BB on the segment ABAB, and since AB=2πrAB = 2\pi r, the mark will be at the top of the ball. Then we roll the ball from BB to OO on BOBO and the mark is at the top of the ball while it is on its first position.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.