# Use double or half angle formulas

#### Elissa89

##### Member
Ok, So with this problem it says to use double angle or half angle formula. I have the formulas in my notes just not sure how to apply them to the problem I feel like I should be using the double formula though. Here's the problem:

cos(2*theta)+sin^2=0

#### Joppy

##### Well-known member
MHB Math Helper
Yes the double-angle formula for $\cos$ would be useful. There are a few different forms but consider that $\cos (2 \theta) = \cos^2 (\theta) - \sin^2 (\theta)$. Substituting in we get,

$\cos^2 (\theta) - \sin^2 (\theta) + \sin^2 (\theta) = \cos^2 (\theta) = 0$.

Do you have any ideas on how to solve for $\theta$?