, so you require . This occurs at , in this case .
I am given the function with the derivative f '(x)=sin(x^2+1). I need to find out how many relative extrema exist in the interval 2<x<4. To do this I know that I first need to find where the derivative is equal to 0. However I don't know where sin(x^2+1)=0 (and how prove it does at these points)