Maths Olympiad Prep

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Problem 1064

AMC 12 late, AIME early
Number theory Difficulty 5.0 Prove it Hong Kong competition problems · Hong Kong

Find all positive integer(s) nn such that n2+32n+8n^2 + 32n + 8 is a perfect square.

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

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Official solution

nn can be 4747 or 1717.

Suppose n2+32n+8=m2n^2 + 32n + 8 = m^2 where mm is a positive integer. Note that this can be rewritten as (n+16)2248=m2(n+16)^2 - 248 = m^2, which implies
(n+m+16)(nm+16)=248=23×31. (n + m + 16)(n - m + 16) = 248 = 2^3 \times 31.
Hence, (n+m+16,nm+16)=(248,1),(124,2),(62,4),(31,8)(n+m+16, n-m+16) = (248, 1), (124, 2), (62, 4), (31, 8). For each pair, we can solve the system of two linear equations in two unknowns. The only positive integer solutions are (n,m)=(47,61),(17,29)(n, m) = (47, 61), (17, 29).

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