Determine all values function reaches local max

Question

Determine all $x > 0$ where function $f(x) = 1 + \int_{-x}^{x}\cos(t^2) dt $ reaches local maximum. I took the derivative of this function and got $f^\prime(x) = 2\cos(x^2)$ but further i put this equal to zero but can't find out the values for which $\cos(x^2) = 0$. It would be a great help if someone tells me how to find the value where function($\cos(x^2) = 0)$?