Get the cos(x) on the other side of the equation with a minus so you get the big square root minus Cos(x) = 0
So you try to find an x for which this f(x) is worth zero. Just by looking at the function I can see its always positive...if you don't believe me try to find a x value for which F(x) is negative, you won't find any.
Just say that there is no x for which this relation is verified or you can calculate the derivative of the function and find the minimum ( should be around 0.5 eye wise). I think the period is huge as well like modulo a billion pie LOL