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...

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