Inverse Function with Differentiation

1. Nov 8, 2005

Soaring Crane

Let f(x) = x^3 + e^x.
Find (f^-1)'(2).
I know how to do everything else except the first step. How do you find the inverse of f(x)? I know the inverse of an exponential function is a logarthmic function, but where do I proceed from here?
Thanks.

2. Nov 8, 2005

benorin

There's a theorem for that

You don't need to find an inverse function to answer this question, you only need to determine the value of its derivative at 2. I am supposing that you know the chain rule:

Let $y=g(x)$ so that $g^{-1}(y)=x$. Recall that y is a function of x, so in differentiating w.r.t. x we apply the chain rule to get $\left( g^{-1} \right)^{\prime}(y)y^{\prime}=1$ but $y=g(x)$
so put $y^{\prime}=g^{\prime}(x)$ and the equation becomes $\left( g^{-1} \right)^{\prime}\left( g(x)\right) g^{\prime}(x)=1$ or $g^{\prime}(x)=\frac{1}{\left( g^{-1} \right)^{\prime}\left( g(x)\right)}$.

In your problem, let $g(x)=f^{-1}(x)$.