MCQ
If $\phi \,(x) = {\log _5}\,{\log _3}\,x;$ then  $\phi '\,(e)$ is equal to
  • A
    $ e\ log\ 5$
  • B
    $- e\ log\ 5$
  • $\frac{1}{{e\,\ln \,5}}$
  • D
    $\frac{log 5}{{e}}$

Answer

Correct option: C.
$\frac{1}{{e\,\ln \,5}}$
c
$\phi(\mathrm{x})=\log _{5} \log _{3} \mathrm{x}$

$\phi(\mathrm{x})=\log _{\mathrm{s}}\left(\frac{\ln \mathrm{x}}{\ln 3}\right)$

$\phi ({\rm{x}}) = {\log _{\rm{s}}}{\rm{lnx}} - {\log _{\rm{s}}}\ln 3$

$\phi(x)=\frac{\ln \ln x}{\ln 5}-\log _{5} \ln 3$

$\phi^{\prime}(x)=\frac{1}{\ln 5} \times \frac{1}{\ln x} \times \frac{1}{x}$

${\phi ^\prime }(e) = \frac{1}{{e\ln S}}$

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