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Number theory Difficulty 4.8 AIME Prove it China

Show that there are only finitely many triples (a,b,c)(a, b, c) of positive integers satisfying the equation abc=2009(a+b+c)abc = 2009(a + b + c).

Solution

There are at most six permutations for any three numbers x,y,zx, y, z. It suffices to show that there are only finitely many triples (a,b,c)(a, b, c), with abca \ge b \ge c, of positive integers satisfying the equation abc=2009(a+b+c)abc = 2009(a + b + c). It follows that abc2009×(3a)abc \le 2009 \times (3a) or bc2009×3=6027bc \le 2009 \times 3 = 6027. Clearly, there are finitely many pairs (b,c)(b, c) of positive integers satisfying the equation bc6027bc \le 6027 and for each fixed pair of integers (b,c)(b, c) there is at most one positive integer aa satisfying the equation abc=2009(a+b+c)abc = 2009(a + b + c) (because it is a linear equation in aa).

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