Maths Olympiad Prep

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Geometry Difficulty 6.7 National olympiad Prove it Estonia

a. There are 2n2n rays marked in a plane, with nn being a natural number. Given that no two marked rays have the same direction and no two marked rays have a common initial point, prove that there exists a line that passes through none of the initial points of the marked rays and intersects with exactly nn marked rays.

b. Would the claim still hold if the assumption that no two marked rays have a common initial point was dropped?

Solution

Consider any circle such that all of the endpoints of the marked rays are inside the circle. Choose a tangent line of the circle that is not parallel to any of the marked rays. Let this tangent line be l0l_0 and let lαl_\alpha be the tangent line we get by rotating l0l_0 counterclockwise by angle α\alpha with respect to the center of the circle. For any α\alpha, let f(α)f(\alpha) be the number of marked rays that intersect with lαl_\alpha. Since l0l_0 and lπl_\pi are two parallel lines and all of the endpoints of the marked rays are between them, we know from the definition of l0l_0 that f(0)+f(π)=2nf(0) + f(\pi) = 2n. Without loss of generality, let f(0)nf(0) \le n and f(π)nf(\pi) \ge n. Because no two rays have the same direction, when α\alpha increases continuously f(α)f(\alpha) can at any time instance only change by at most one. Thus f(α)f(\alpha) ranges over all integral values between f(0)f(0) and f(π)f(\pi). Hence f(α)=nf(\alpha) = n for some α\alpha.

The argument above does not use the assumption that the initial points of the marked rays are distinct, whence it solves both parts of the problem.

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