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Question: Answered & Verified by Expert
If $f(x)=\frac{2 x}{4+3|x|}, x \in R$, then $f^{\prime}(0)=$
MathematicsContinuity and DifferentiabilityTS EAMCETTS EAMCET 2018 (05 May Shift 2)
Options:
  • A 0
  • B $\frac{1}{4}$
  • C $\frac{1}{2}$
  • D $\frac{3}{4}$
Solution:
2878 Upvotes Verified Answer
The correct answer is: $\frac{1}{2}$
We have, $f(x)=\frac{2 x}{4+3|x|}, x \in R$
$$
\begin{aligned}
& = \begin{cases}\frac{2 x}{4+3 x}, & \text { if } x \geq 0 \\
\frac{2 x}{4-3 x}, & \text { if } x < 0\end{cases} \\
\therefore \quad f^{\prime}(x) & = \begin{cases}\frac{(4+3 x) \cdot 2-2 x \cdot 3}{(4+3 x)^2}, & \text { if } x \geq 0 \\
\frac{(4-3 x) \cdot 2-2 x(-3)}{(4-3 x)^2}, & \text { if } x < 0\end{cases}
\end{aligned}
$$


$$
\begin{aligned}
& = \begin{cases}\frac{8+6 x-6 x}{(4+3 x)^2}, & \text { if } x \geq 0 \\
\frac{8-6 x+6 x}{(4-3 x)^2}, & \text { if } x < 0\end{cases} \\
& = \begin{cases}\frac{8}{(4+3 x)^2}, & \text { if } x \geq 0 \\
\frac{8}{(4-3 x)^2}, & \text { if } x < 0\end{cases}
\end{aligned}
$$
Now, $L f^{\prime}(0)=\lim _{x \rightarrow 0^{-}} \frac{8}{(4-3 x)^2}=\frac{8}{16}=\frac{1}{2}$
$$
\text { and } R f^{\prime}(0)=\lim _{x \rightarrow 0^{+}} \frac{8}{(4+3 x)^2}=\frac{8}{16}=\frac{1}{2}
$$
Since, $\quad L f^{\prime}(0)=R f^{\prime}(0)$
$$
\therefore \quad f^{\prime}(0)=\frac{1}{2}
$$

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