### 40th Vietnam 2002 problems A1.  Solve the following equation: √(4 - 3√(10 - 3x)) = x - 2. A2.  ABC is an isosceles triangle with AB = AC. O is a variable point on the line BC such that the circle center O radius OA does not have the lines AB or AC as tangents. The lines AB, AC meet the circle again at M, N respectively. Find the locus of the orthocenter of the triangle AMN. A3.  m < 2001 and n < 2002 are fixed positive integers. A set of distinct real numbers are arranged in an array with 2001 rows and 2002 columns. A number in the array is bad if it is smaller than at least m numbers in the same column and at least n numbers in the same row. What is the smallest possible number of bad numbers in the array? B1.  If all the roots of the polynomial x3 + a x2 + bx + c are real, show that 12ab + 27c ≤ 6a3 + 10(a2 - 2b)3/2. When does equality hold? B2.  Find all positive integers n for which the equation a + b + c + d = n√(abcd) has a solution in positive integers. B3.  n is a positive integer. Show that the equation 1/(x - 1) + 1/(22x - 1) + ... + 1/(n2x - 1) = 1/2 has a unique solution xn > 1. Show that as n tends to infinity, xn tends to 4. 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
21 Nov 2002