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Algebra Difficulty 5.8 AIME, harder Find the answer

4. 98 Find all integer solutions of the following equation:
cos(π8(3x9x2+160x+800))=1.\cos \left(\frac{\pi}{8}\left(3 x-\sqrt{9 x^{2}+160 x+800}\right)\right)=1 .

A number or a short expression. Spacing and $ signs are ignored.

Solution

[Solution] Let xx be an integer solution of the original equation, then there must exist an integer nn such that
π8(3x9x2+160x+800)=2nπ\frac{\pi}{8}\left(3 x-\sqrt{9 x^{2}+160 x+800}\right)=2 n \pi

i.e., \square
9x2+160x+800=(3x16n)2x(3n+5)=8n225\begin{array}{l} 9 x^{2}+160 x+800=(3 x-16 n)^{2} \\ x(3 n+5)=8 n^{2}-25 \end{array}

However,
Therefore,
8n225=89(3n+5)(3n5)2598 n^{2}-25=\frac{8}{9}(3 n+5)(3 n-5)-\frac{25}{9}
x(3n+5)=89(3n+5)(3n5)2598(3n+5)(3n5)9x(3n+5)=25(3n+5)(24n409x)=253n+5{±1,±5,±25}\begin{array}{l} x(3 n+5)=\frac{8}{9}(3 n+5)(3 n-5)-\frac{25}{9} \\ 8(3 n+5)(3 n-5)-9 x(3 n+5)=25 \\ (3 n+5)(24 n-40-9 x)=25 \\ 3 n+5 \in\{ \pm 1, \pm 5, \pm 25\} \end{array}

Noting that nn is an integer, we get
n{2,0,10}n \in\{-2,0,-10\}

Substituting n=2n=-2 into (1) gives x=7\quad x=-7;
Substituting n=0n=0 into (1) gives x=5\quad x=-5;
Substituting n=10n=-10 into (1) gives x=31\quad x=-31.
Upon verification, x=5x=-5 is an extraneous root. The integer solutions of the original equation are x=7x=-7 or x=31x=-31.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.