If $f(1)=3$, $f'(1)=2$, $f''(1)=4$, then $(f^{-1})''(3) =$

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I tried using the inverse function theorem, but the answer I got through that was $-1$.

The answer key says the answer is $-1/2$.

Where am I going wrong?

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Let $g(x)=f^{-1}(x)$

$$ f(g(x))=x\Rightarrow f'(g(x))g'(x)=1\Rightarrow g'(x)={1\over f'(g(x))} $$

$$ g''(x)=-{f''(g(x))g'(x)\over (f'(g(x)))^2}=-{f''(g(x))\over (f'(g(x)))^3} $$

Substituting values

$$ g''(3)=-{f''(g(3))\over (f'(g(3)))^3}=-{f''(1)\over (f'(1))^3}=-{4\over8}=-{1\over2} $$