Question

Rewrite with only sin x and cos x.

cos 3x

Answer 1

http://en.wikipedia.org/wiki/List_of_trigonometric_identities

^ Know those well. The rest is easy.

Answer 2

@Ahaanomegas Can you explain it to me?

Answer 3

Sure. What would you like an explanation of? Do you have the formulas down?

Answer 4

@Ahaanomegas No I do not have the formulas down.

Answer 5

You should look at them in the Wiki link I posted above.

Answer 6

@Ahaanomegas I don't know which one to use

Answer 7

Ah! cos (3x) = cos (2x + x) and cos (2x) is a double-angle usage. You can derive the double-angle formula if you like by using the same logic -> cos (2x) = cos (x + x). Try that!

Answer 8

@Ahaanomegas can you show me how to do it step by step