# Math Help - Help with trig functions please

1. ## Help with trig functions please

I need to know how

$1-\cos(2\arcsin\frac{s}{4}) = \frac{s^{2}}{8}$

and how

$\sin(2\arcsin\frac{s}{4}) = 2\frac{s}{4}\sqrt{1-\frac{s^{2}}{16}}$

I know the rule $\cos\arcsin(x)) = \sqrt{1-x^{2}}$ but don't know what to do with the 2 before the inverse trig functions eah time.

Thanks

2. Originally Posted by bobby
I need to know how

$1-\cos(2\arcsin\frac{s}{4}) = \frac{s^{2}}{8}$
You need the identity $\cos(2\theta)= \cos^2(\theta)- \sin^2(\theta)$ and replacing $\cos^2(\theta)$ by $1- \sin^2(\theta)$, $\cos(2\theta)= 1- 2\sin^2(\theta)$.

So $1- \cos(2\arcsin(\frac{s}{4})= 1- 1+ 2\sin^2(\arcsin(\frac{s}{4}))$ $= 2s\\in^2(\arcsin(\frac{s}{4}))$.

Surely you know what $sin(\arcsin(\frac{s}{4})$ is?

and how

$\sin(2\arcsin\frac{s}{4}) = 2\frac{s}{4}\sqrt{1-\frac{s^{2}}{16}}$
$\sin(2\theta)= 2 \sin(\theta)cos(\theta)$

I know the rule $\cos\arcsin(x)) = \sqrt{1-x^{2}}$ but don't know what to do with the 2 before the inverse trig functions eah time.

Thanks

3. Thanks very much HallsofIvy