What is the Inverse Derivative of f(x)=x^3+x at x=2?

[Mach]

Homework Statement

Assume the function f(x)=x^3+x has an inverse in R. Determine d/dx(f^-1(x)) at x=2.

Homework Equations

(f^-1(x))'=1/f'(f^-1(x))

The Attempt at a Solution

y'=3x^2+1

(f^-1(x))'=1/(3y^2+1)

now i substitute f^-1(2), y=10, into the equation but do not get the correct answer. I suspect that the mistake i am making is because i do not fully understand the question. Any help is greatly appreciated.

 

[ZioX]

f^-1 is the functional inverse, not the reciprocal.

 

[nizi]

<br /> \frac{dy}{dx} = 3 x^2 + 1<br />
using implicit differentiaion
<br /> \frac{dx}{dy} = \frac{1}{\frac{dy}{dx}} = \frac{1}{3 x^2 + 1}<br />
accordingly replacing x by y
<br /> \frac{dy}{dx} = \frac{1}{3 y^2 + 1}<br />
i.e.
<br /> \frac{dy}{dx} = \frac{1}{3 \left( {x^3 + x } \right)^2 + 1}<br />
then you have only to substituting x=2.