trigo problem: solve cos(@) + cos(3@) = 0

[xxxxleungxxxx]

i don't really know how to solve this problem
cos ? + cos 3? = 0
so, cos ? = - (cos 3?)
then what should i do to solve it?
thanks

 

[royhaas]

Expand $\displaystyle cos(3 \theta)$ as a function of $\displaystyle cos(\theta)$.

 

[Loren]

I would use the identity...

$\displaystyle \cos x + \cos y = 2\cos(\frac{x+y}{2})\cos(\frac{x-y}{2})$