differentiation with Inverse function.

stuart clark

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Mar 3, 2011
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If \(\displaystyle f(x) = (2x-\pi)^3+2x-cosx\).Then Calculate \(\displaystyle \displaystyle \frac{d}{dx}\left(f^{-1}(x)\right)\) at \(\displaystyle x=\pi\)
 
If the given function is complicated to solve in order to obtain an inverse, you can use:

\(\displaystyle \frac{dy}{dx}=\frac{1}{\frac{dx}{dy}}\)

\(\displaystyle x=(2y-\pi)^{3}+2y-cos(y)\)

\(\displaystyle \frac{dx}{dy}=6(2y-\pi)^{2}+2+sin(y)\)

\(\displaystyle \frac{dy}{dx}=\frac{1}{\frac{dx}{dy}}=\frac{1}{6(2y-\pi)^{2}+2+sin(y)}\)

Leave it as is. It is too complicated to solve for y in terms of x.

Or, implicit differentiation can be used:

\(\displaystyle \frac{d}{dx}[x]=\frac{d}{dx}[(2y-\pi)^{3}+2y-cos(y)]\)

\(\displaystyle 1=(6(2y-\pi)^{2}\frac{dy}{dx}+2+sin(y))\frac{dy}{dx}\)

\(\displaystyle \frac{dy}{dx}=\frac{1}{6(2y-\pi)^{2}+2+sin(y)}\)

Which agrees with the first case.
 
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