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CEMC Fermat, 2013

Number theory Difficulty 2.6 Junior Find the answer Canada

If mm, nn and pp are positive integers with m+1n+1p=173m + \dfrac{1}{n+\dfrac{1}{p}} = \dfrac{17}{3}, the value of nn is

Pick one

Solution

Since pp is a positive integer, then p≥1p \geq 1 and so 0 lt; 1 p 1\text{0 lt; 1 p 1}.

Since nn is a positive integer, then n≥1n \geq 1 and so n + 1 p gt;1\text{n + 1 p gt;1}, which tells us that 0 lt; 1 n+ 1 p lt; 1\text{0 lt; 1 n+ 1 p lt; 1}.

Therefore, m lt; m + 1 n+ 1 p lt; m+1\text{m lt; m + 1 n+ 1 p lt; m+1}. Since m+1n+1p=173m + \dfrac{1}{n+\dfrac{1}{p}} = \dfrac{17}{3}, which is between 5 and 6, and since mm is an integer, then m=5m=5.

Since m=5m=5, then m+1n+1p=173m + \dfrac{1}{n+\dfrac{1}{p}} = \dfrac{17}{3} gives 1n+1p=23\dfrac{1}{n+\dfrac{1}{p}} = \dfrac{2}{3} or n+1p=32n+\dfrac{1}{p} = \dfrac{3}{2}.

Since n lt; n+ 1 p n+1\text{n lt; n+ 1 p n+1} and nn is an integer, then n=1n=1.

Thus, n+1p=32n+\dfrac{1}{p} = \dfrac{3}{2} gives 1p=12\dfrac{1}{p} = \dfrac{1}{2}, which gives p=2p=2.

Therefore, n=1n=1.

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