Formulas:
$\sin (2A) = 2\sin (A) \cos (A)$
$\cos (2A) = \cos^2 (A) − \sin^2 (A) = 1 − 2 \sin^2 (A) = 2\cos^2 (A) − 1$
$\tan (2A) = \dfrac{2\tan (A)}{1 − \tan^2 (A)}$
I'm really confused, if I'm supposed to expand $\cos(4x)$ using a double angle formula. how do I know which formula to use? This is just one of the questions, the same goes for $\sin(4x)$ and $\sin(6x)$. Is there a method I can use to find out which formula should I use?
for $\cos(4x)$ I used $2\cos^2(2x)-1$ but that formula wasn't used in the answers.
Best Answer
Whatever formula you use, the answer will be right, unless additional instructions were given.