Hmmm...no - I don't think so. Because if you differentiate what you thought it was - you have a derivate of one over f'(x) to think about. If that didn't involve x, you would be ok, but in this case it does. I think you have to use the reduction formula...although I don't think it'll work completely because you've got a square root involved...I'm a bit rusty on all this though!