Question

Using power series expansion for cosx, write down the first three terms of the power series expansion for cos(3x) and for cos(x/3)

Answer 1

Just write out cos(x)'s terms out and plug in a 3x or a x/3 everywhere you see x.

Answer 2

oh so cosx = 1 -x^2/2! + x^4/4! etc

Answer 3

Yep.