4. 98 Find all integer solutions of the following equation: cos(8π(3x−9x2+160x+800))=1.
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Solution
[Solution] Let x be an integer solution of the original equation, then there must exist an integer n such that 8π(3x−9x2+160x+800)=2nπ
i.e., □ 9x2+160x+800=(3x−16n)2x(3n+5)=8n2−25
However, Therefore, 8n2−25=98(3n+5)(3n−5)−925 x(3n+5)=98(3n+5)(3n−5)−9258(3n+5)(3n−5)−9x(3n+5)=25(3n+5)(24n−40−9x)=253n+5∈{±1,±5,±25}
Noting that n is an integer, we get n∈{−2,0,−10}
Substituting n=−2 into (1) gives x=−7; Substituting n=0 into (1) gives x=−5; Substituting n=−10 into (1) gives x=−31. Upon verification, x=−5 is an extraneous root. The integer solutions of the original equation are x=−7 or x=−31.
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Source: NuminaMath-1.5,
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