Practicing Success

Target Exam

CUET

Subject

-- Mathematics - Section B1

Chapter

Continuity and Differentiability

Question:

If $f(x)=|\log _e|x||$, then f'(x) equals

Options:

$\frac{1}{|x|}, x \neq 0$

$\frac{1}{x}$ for $|x|>1$ and $-\frac{1}{x}$ for $|x|<1$

$-\frac{1}{x}$ for $|x|>1$ and $\frac{1}{x}$ for $|x|<1$

$\frac{1}{x}$ for $x>0$ and $-\frac{1}{x}$ for $x<0$

Correct Answer:

$\frac{1}{x}$ for $|x|>1$ and $-\frac{1}{x}$ for $|x|<1$

Explanation:

For x > 1, we have

$f(x)=|\log | x||=\log x \Rightarrow f'(x)=\frac{1}{x}$

For x < -1, we have

$f(x)=|\log |x||=\log (-x) \Rightarrow f'(x)=\frac{1}{x}$

For 0 < x < 1, we have

$f(x)=|\log |x||=-\log x \Rightarrow f'(x)=-\frac{1}{x}$

For -1< x < 0, we have

$f(x)=-\log (-x) \Rightarrow f'(x)=-\frac{1}{x}$

Hence, $f'(x)=\left\{\begin{aligned} \frac{1}{x}, & |x|>1 \\ -\frac{1}{x}, & |x|<1\end{aligned}\right.$