Math Help - I figured out the half angle just need help with the math

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

Given and is in quadrant III, find the exact value of .
Note: You are not allowed to use decimals in your answer. =
$\cos(\frac{\alpha}{2})=-\sqrt\frac{1+-(\frac{4}{5})}{2}$
I need help with the math after this??

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

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

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

That worked perfectly
$-\sqrt\frac{1}{10}$ is just what I needed