Maths Olympiad Prep

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Algebra Difficulty 4.8 AIME Find the answer

Find all positive integers n>1n>1 for which n2+7n+136n1\frac{n^{2}+7 n+136}{n-1} is the square of a positive integer.

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Write n2+7n+136n1=n+8n+136n1=n+8+144n1=9+(n1)+144(n1)\frac{n^{2}+7 n+136}{n-1}=n+\frac{8 n+136}{n-1}=n+8+\frac{144}{n-1}=9+(n-1)+\frac{144}{(n-1)}. We seek to find pp and qq such that pq=144p q=144 and p+q+9=k2p+q+9=k^{2}. The possibilities are seen to be 1+144+9=154,2+72+9=83,3+48+9=60,4+36+9=49,6+24+9=391+144+9=154,2+72+9=83,3+48+9=60,4+36+9=49,6+24+9=39, 8+18+9=35,9+16+9=348+18+9=35,9+16+9=34, and 12+12+9=3312+12+9=33. Of these, {p,q}={4,36}\{p, q\}=\{4,36\} is the only solution to both equations. Hence n1=4,36n-1=4,36 and n=5,37n=5,37.

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