12th USAMO 1983

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1.  If six points are chosen sequentially at random on the circumference of a circle, what is the probability that the triangle formed by the first three is disjoint from that formed by the second three.
2.  Show that the five roots of the quintic a5x5 + a4x4 + a3x3 + a2x2 + a1x + a0 = 0 are not all real if 2a42 < 5a5a3.
3.  S1, S2, ... , Sn are subsets of the real line. Each Si is the union of two closed intervals. Any three Si have a point in common. Show that there is a point which belongs to at least half the Si.
4.  Show that one can construct (with ruler and compasses) a length equal to the altitude from A of the tetrahedron ABCD, given the lengths of all the sides. [So for each pair of vertices, one is given a pair of points in the plane the appropriate distance apart.]
5.  Prove that an open interval of length 1/n in the real line contains at most (n+1)/2 rational points p/q with 1 ≤ q ≤ n.

To avoid possible copyright problems, I have changed the wording, but not the substance, of the problems.

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© John Scholes
jscholes@kalva.demon.co.uk
25 Aug 2002