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

**M670** · November 24th 2012, 07:58 PM

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??

**jakncoke** · November 24th 2012, 08:10 PM

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

**M670** · November 24th 2012, 08:17 PM

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

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