# Math Help - trig equation

1. ## trig equation

Find all values of theta in the interval 0<theta<360

2 cos^2 theta - 1=0

2. Originally Posted by blame_canada100
Find all values of theta in the interval 0<theta<360

2 cos^2 theta - 1=0
$2\cos ^2 \theta -1 = 0$

$\cos ^2 \theta = \frac{1}{2}$

$\cos \theta = \mp \frac{1}{\sqrt{2}}$

theta equal the angle 45 in all quarters

Originally Posted by blame_canada100
Find all values of theta in the interval 0<theta<360

2 cos^2 theta - 1=0
Alternatively, you could use $\cos2\theta = 2\cos^2\theta - 1$

So $\cos2\theta = 0$

$\Rightarrow 2\theta = 90^o,\,270^o,\,450^o,\,630^o$

Then divide by $2$.