# Solve sin4(theta) = cos2(theta)

• December 8th 2008, 10:07 AM
dno
Solve sin4(theta) = cos2(theta)
dont worry about the values im just having trouble setting it up
• December 8th 2008, 10:23 AM
TwistedOne151
Use the double angle identity $\sin(4\theta)=2\sin(2\theta)\cos(2\theta)$.
This gives:
$\sin(4\theta)=\cos(2\theta)$
$2\sin(2\theta)\cos(2\theta)=\cos(2\theta)$
$2\sin(2\theta)\cos(2\theta)-\cos(2\theta)=0$
$\left(2\sin(2\theta)-1\right)\cos(2\theta)=0$
You should be able to see how to solve it from there.

--Kevin C.