# Thread: trigonometric multiple angle formula

1. ## trigonometric multiple angle formula

Hi could I get Help with this question really struggling I need to use angle formula to show cos^2x=1/2 (cos2x-1).

2. ## Re: trigonometric multiple angle formula

Which "angle formula" are you supposed to use? Are you talking about the double angle formula for cosine?

$\displaystyle \cos(2x) = 2\cos^2 x -1$

Is that the "angle formula" you are referring to? If it is, then add 1 to both sides. Then divide both sides by 2.

3. ## Re: trigonometric multiple angle formula

I haven't been given one I've been told to use one of the multiple angle formula as to which one to use I'm not quite sure

4. ## Re: trigonometric multiple angle formula

$\cos(\alpha + \beta) = \cos \alpha \cos \beta - \sin \alpha \sin \beta$

Let $\alpha = x$ and $\beta=x$. See what you get on the left and right sides.

5. ## Re: trigonometric multiple angle formula

It should be $\cos^2(x)=(1/2) (\cos(2x)\large\color{red}+1)$