You're correct, and the answer's correct too, because cos(-x)=cos(x)

Yes, but you can write cos(2x)=1-2sinČ(x) and sin(2x)=2sin(x)cos(x)2)express sin(3x) in terms of sin(x). This questions follows on from the previous question which was asking to expand sin(x+2x) answer to this is .

and then use cosČ(x)=1-sinČ(x) to simplify and get something with respect to sin(x) only.

Multiply your answer by (multiplying the numerator and the denominator by the conjugate of the denominator - it's similar to 'rationalizing the denominator')3)Use the compound angle formulas and appropriate angles to find the exact value of .

This is what i have done:

book's answer is

and use the identities : (a-b)(a+b)=aČ-bČ and (a-b)Č=aČ+bČ-2ab