So we know from the definition of the derivative that the derivative of the function square root of x, that is equal to, let me switch colors just for variety, that's equal to the limit as delta x approaches 0. And some people say h approaches 0 or d approaches 0. I just use delta x. So the change in x approaches 0. And then we say f of x plus delta x. So in this case, this is f of x. So it's the square root of x plus delta x minus f of x, in this case, the square root of x.