a. There are 2n rays marked in a plane, with n 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 n 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 l0 and let lα be the tangent line we get by rotating l0 counterclockwise by angle α with respect to the center of the circle. For any α, let f(α) be the number of marked rays that intersect with lα. Since l0 and lπ are two parallel lines and all of the endpoints of the marked rays are between them, we know from the definition of l0 that f(0)+f(π)=2n. Without loss of generality, let f(0)≤n and f(π)≥n. Because no two rays have the same direction, when α increases continuously f(α) can at any time instance only change by at most one. Thus f(α) ranges over all integral values between f(0) and f(π). Hence f(α)=n for some α.
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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Source: MathNet,
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