MCQ
If $\text{f(x)}=|\text{x}-\text{a}|\ \phi\ (\text{x}),$ where $\phi(\text{x})$ is continuous function, then:
  • A
    $\text{f}'(\text{a}^+)=\phi(\text{a})$
  • $\text{f}'(\text{a}^-)=-\phi(\text{a})$
  • C
    $\text{f}'(\text{a}^+)=\text{f}'(\text{a}^-)$
  • D
    None of these

Answer

Correct option: B.
$\text{f}'(\text{a}^-)=-\phi(\text{a})$
Given that $\text{f(x)}=|\text{x}-\text{a}|\ \phi\ (\text{x}),$ where $\phi(\text{x})$ continuous function.

$|\text{x}-\text{a}|\Rightarrow\text{x}-\text{a}$ if $\text{x}-\text{a}>0$

$|\text{x}-\text{a}|\Rightarrow-(\text{x}-\text{a})$ if $\text{x}-\text{a}<0$

By definition of continuity,

$\text{f}'(\text{a})= \lim\limits_{\text{h}\rightarrow0}\frac{\text{f}(\text{a}+\text{h})-\text{f(a)}}{\text{h}}$

Hence, $\text{f}(\text{a}^+)=\phi(\text{x})$ and $\text{f}'(\text{a}^-)=-\phi(\text{x})$

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