Question
If $y=\log \left[\cos \left(x^5\right)\right]$ then find $\frac{ d y}{ d x}$

Answer

$y=\log \left[\cos \left(x^5\right)\right]$
Differentiating w.r.t. $x$, we get
$ \frac{ d y}{ d x}=\frac{ d }{ d x}\left[\log \left\{\cos \left(x^5\right)\right\}\right]$
$=\frac{1}{\cos \left(x^5\right)} \cdot \frac{ d }{ d x}\left[\cos \left(x^5\right)\right]$
$=\frac{1}{\cos \left(x^5\right)} \cdot\left[-\sin \left(x^5\right)\right] \cdot \frac{ d }{ d x}\left(x^5\right)$
$=\frac{-\sin \left(x^5\right)}{\cos \left(x^5\right)} \cdot 5 x^4$
$=-5 x^4 \tan \left(x^5\right) $

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