## butterflyprincess 2 years ago H E L P : Cos (13pi/12 ) Use Half Angle ID to simplify ~

1. butterflyprincess

I got : -root 3 - root 6 / 4 Is that what you got..?

2. Venomblast

3. butterflyprincess

What do you mean? You just use that ID to simplify?

4. Venomblast

but i dont what ID.

5. jim_thompson5910

$\Large \cos\left(\frac{13\pi}{12}\right) = \cos\left(\frac{1}{2}\cdot\frac{13\pi}{6}\right)$ $\Large \cos\left(\frac{13\pi}{12}\right) = -\sqrt{\frac{1+\cos\left(\cdot\frac{13\pi}{6}\right)}{2}}$ $\Large \cos\left(\frac{13\pi}{12}\right) = -\sqrt{\frac{1+\frac{\sqrt{3}}{2}}{2}}$ $\Large \cos\left(\frac{13\pi}{12}\right) = -\sqrt{\frac{1}{2}+\frac{\sqrt{3}}{4}}$ $\Large \cos\left(\frac{13\pi}{12}\right) = -\sqrt{\frac{2}{4}+\frac{\sqrt{3}}{4}}$ $\Large \cos\left(\frac{13\pi}{12}\right) = -\sqrt{\frac{2+\sqrt{3}}{4}}$ $\Large \cos\left(\frac{13\pi}{12}\right) = -\frac{\sqrt{2+\sqrt{3}}}{\sqrt{4}}$ $\Large \cos\left(\frac{13\pi}{12}\right) = -\frac{\sqrt{2+\sqrt{3}}}{2}$

6. butterflyprincess

cool thnx! (:

7. jim_thompson5910

yw