+0

# Halp

-2
42
1

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 $a + b + c + d.$

off-topic
Mar 31, 2020
edited by Guest  Mar 31, 2020