Find the values of x at the stationary points for sin(x^2)

I found the 1st derivative which is 2xcos(x^2)

I know the final answer is x = 0 and x = \pm sqrt((k+(1/2))pi) for k = 0,1,2....

How do you get that answer?? I can see how the answer is x = 0, but how do you figure out the other one?