# Calc II question help

1. Oct 1, 2007

### frasifrasi

This deals with inverse functions:

suppose g(x) is the inverse of f(X) and G(X) = 1/g(X). If f(3) = 3 and f'(3) = 1/9, find G'(3).

Does anyone know how to answer this question?

Thanks.

I was thinking of using the formula g'(x) = 1/f'(g(X)), but the G(X) is throwing me off.

2. Oct 1, 2007

### ptr

$$f'(x)= \frac{1}{f^{-1}'(x)}$$

3. Oct 1, 2007

### frasifrasi

tthat is not what is being asked...

4. Oct 1, 2007

### HallsofIvy

Staff Emeritus
If G(x)= 1/ g(x)= g-1 then, by the chain rule, G'= -1g(x)-2 g'(x). Since g(x) is f-1(x), g'(x)= 1/f'(x).

5. Oct 1, 2007

### frasifrasi

I still don't get it. Can anyone explain it using the actual numbers to derive an answer? I need to understand this before the exam.