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There is some kind of formula here, half angle, or some such that I forget, but I do remember the algorithm. So...,
int[cos(10X)cos(15X)] dx
since this is multiplicative, switch it around
int[cos(15X)cos(10X)] dx
int[cos(15X - 10X)/2(15 -10) + cos(15X + 10X)/2(15 + 10)] dx
int[cos(5X)/10 + cos(25X)/50] dx
= 1/10sin(5X) + 1/50sin(25X) + C
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