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The 15th Vietnam Mathematical Olympiad
1977年第十五届越南数学奥林匹克

A1. Find all real x such that √(x - 1/x) + √(1 - 1/x) > (x - 1)/x.

A2. Show that there are 1977 non-similar triangles such that the angles A, B, C satisfy (sin A + sin B + sin C)/(cos A + cos B + cos C) = 12/7 and sin A sin B sin C = 12/25.

A3. Into how many regions do n circles divide the plane, if each pair of circles intersects in two points and no point lies on three circles?

B1. p(x) is a real polynomial of degree 3. Find necessary and sufficient conditions on its coefficients in order that p(n) is integral for every integer n.

B2. The real numbers a0, a1, ... , an+1 satisfy a0 = an+1 = 0 and | ak-1 - 2ak + ak+1| ≤ 1 for k = 1, 2, ... , n. Show that |ak| ≤ k(n + 1 - k)/2 for all k.

B3. The planes p and p' are parallel. A polygon P on p has m sides and a polygon P' on p' has n sides. Find the largest and smallest distances between a vertex of P and a vertex of P'.

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