If n and m are both even,
then the tricks of the previous slide do not help. Instead,
we can either integrate by parts (using the "go in a circle" trick
in the previous module) or use double-angle formulas. Most
people find the double-angle formulas to be easier, and that's
what this slide's video is about.
Remember the trig
identities we said would be helpful: sin2(x)=1−cos(2x)2
cos2(x)=1+cos(2x)2.
This converts sinn(x)cosm(x),
with n and m even, into terms of cos(2x). The odd
powers of cos(2x) can be handled as in the previous
slide. To integrate the even powers of cos(2x), apply
the double-angle trick again, getting a polynomial in
cos(4x). Repeat as many times as necessary. While
this can be tedious, it is not hard.