Maths Olympiad Prep

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Number theory Difficulty 5.7 AIME, harder Prove it Philippines

Problem:
How many ordered pairs (x,y)(x, y) of positive integers, where x<yx < y, satisfy the equation
1x+1y=12007 \frac{1}{x} + \frac{1}{y} = \frac{1}{2007}

Solution

Solution:
We can rewrite the given equation into
(x2007)(y2007)=20072=342232 (x - 2007)(y - 2007) = 2007^{2} = 3^{4} \cdot 223^{2}
Since x<yx < y, we have x2007<y2007x - 2007 < y - 2007. It follows that
2007<x2007<2007orx2007<2007 -2007 < x - 2007 < 2007 \quad \text{or} \quad |x - 2007| < 2007
Thus, we have y2007>2007|y - 2007| > 2007.
x2007y20071342232333223232322232333223234223222334223322333223 \begin{array}{rl} x - 2007 & y - 2007 \\ \hline 1 & 3^{4} \cdot 223^{2} \\ 3 & 3^{3} \cdot 223^{2} \\ 3^{2} & 3^{2} \cdot 223^{2} \\ 3^{3} & 3 \cdot 223^{2} \\ 3^{4} & 223^{2} \\ 223 & 3^{4} \cdot 223 \\ 3 \cdot 223 & 3^{3} \cdot 223 \end{array}
For every pair of values of x2007x - 2007 and y2007y - 2007 in the above table, there is a corresponding pair of xx and yy. Thus, there are seven such ordered pairs.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.