MCQ
The function $f(x) = e^x + x$, being differentiable and one to one, has a differentiable inverse $f^{-1} (x)$. The value of $(f^{-1})$ at the point $f(l n2)$ is
  • A
    $\frac{1}{{\ell n2}}$
  • $\frac{1}{3}$
  • C
    $\frac{1}{4}$
  • D
    none

Answer

Correct option: B.
$\frac{1}{3}$
b
$y = e^x + x ;$ diff. w.r.t y,

$1 = (e^x + 1) \frac{{dx}}{{dy}};\frac{{dx}}{{dy}} =\frac{1}{{{e^x} + 1}} =\frac{1}{{{e^x} + 1}}$

$=>{\left. {\frac{{dx}}{{dy}}} \right]_{x = \ell n2}} =\frac{1}{{{e^{\ell n2}} + 1}}$ 

Alternate : $\frac{{dy}}{{dx}}\,\, = \,{e^x} + 1\,\,;\,\,{\left. {\frac{{dy}}{{dx}}\,} \right|_{x = \ell n\,2}}\,\, = \,\,3\,\,\, \Rightarrow \,\,\frac{{dx}}{{dy}}\,\,\,\,\, = \,\,\frac{1}{3}$ 

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