Maths Olympiad Prep

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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 p1p \geq 1 and so 0<1p10<\dfrac{1}{p}\leq 1.

Since nn is a positive integer, then n1n \geq 1 and so n+1p>1n + \dfrac{1}{p}>1, which tells us that 0<1n+1p<10 < \dfrac{1}{n+\dfrac{1}{p}} < 1.

Therefore, m<m+1n+1p<m+1m < m + \dfrac{1}{n+\dfrac{1}{p}} < 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<n+1pn+1n < n+\dfrac{1}{p} \leq 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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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.