Discuss the continuity and differentiability


Discuss the continuity and differentiability of $f(x)=e^{|x|}$.


Given: $f(x)=e^{|x|}$

$\Rightarrow f(x)= \begin{cases}e^{x}, & x \geq 0 \\ e^{-x}, & x<0\end{cases}$


(LHL at x = 0) 

$\lim _{x \rightarrow 0^{-}} f(x)$

$=\lim _{h \rightarrow 0} f(0-h)$

$=\lim _{h \rightarrow 0} e^{-(0-h)}$

$=\lim _{h \rightarrow 0} e^{h}$



$(\mathrm{RHL}$ at $x=0)$

$\lim _{x \rightarrow 0^{+}} f(x)$

$=\lim _{h \rightarrow 0} f(0+h)$

$=\lim _{h \rightarrow 0} e^{(0+h)}$



and $f(0)=e^{0}=1$

Thus, $\lim _{x \rightarrow 0^{-}} f(x)=\lim _{x \rightarrow 0^{+}} f(x)=f(0)$

Hence,function is continuous at x = 0 .

Differentiability at = 0.

(LHD at $x=0$ )

$\lim _{x \rightarrow 0^{-}} \frac{f(x)-f(0)}{x-0}$

$=\lim _{h \rightarrow 0} \frac{f(0-h)-f(0)}{0-h-0}$

$=\lim _{h \rightarrow 0} \frac{e^{-(0-h)}-1}{-h}$


$=\lim _{h \rightarrow 0} \frac{e^{h}-1}{-h}$


$\left[\because \lim _{h \rightarrow 0} \frac{e^{h}-1}{h}=1\right]$

$(\mathrm{RHD}$ at $x=0)$

$\lim _{x \rightarrow 0^{+}} \frac{f(x)-f(0)}{x-0}$

$=\lim _{h \rightarrow 0} \frac{f(0+h)-f(0)}{0+h-0}$

$=\lim _{h \rightarrow 0} \frac{e^{(0+h)}-1}{h}$


$=\lim _{h \rightarrow 0} \frac{e^{h}-1}{h}$

$=1$          $\left[\because \lim _{h \rightarrow 0} \frac{e^{h}-1}{h}=1\right]$

LHD at $(x=0) \neq R H D$ at $(x=0)$

Hence the function is not differentiable at $x=0$.

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