I figured out the half angle just need help with the math

Given http://webwork.mathstat.concordia.ca...d4cc3990b1.png and http://webwork.mathstat.concordia.ca...31d28e5821.png is in quadrant III, find the exact value of http://webwork.mathstat.concordia.ca...f43f6e9341.png.

Note: You are not allowed to use decimals in your answer. http://webwork.mathstat.concordia.ca...cee78702d1.png =

$\displaystyle \cos(\frac{\alpha}{2})=-\sqrt\frac{1+-(\frac{4}{5})}{2}$

I need help with the math after this??

Re: I figured out the half angle just need help with the math

$\displaystyle 1 - \frac{4}{5} = \frac{5 - 4}{5} = \frac{1}{5} $ bring the 5 down. You get $\displaystyle -\sqrt{\frac{1}{2 * 5 }} $

Re: I figured out the half angle just need help with the math

That worked perfectly

$\displaystyle -\sqrt\frac{1}{10}$ is just what I needed