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The expression $\cos x + \cos 3x + \cos 7x + \cos 9x$ can be written in the equivalent form \[a \cos bx \cos cx \cos dx\]for some positive integers $a,$ $b,$ $c,$ and $d.$ Find $a + b + c + d.$

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 Mar 31, 2020
edited by Guest  Mar 31, 2020
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a + b + c + d = 2 + 5 + 8 + 9 = 24.

 Mar 31, 2020

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