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JEE Advanced 2015Paper 1
Exam
JEE Advanced
Year
2015
Paper
Paper 1
Subject
Mathematics
Chapter
Continuity and Differentiability

JEE Advanced 2015 · Paper 1 · Mathematics · Continuity and Differentiability

Let g: ℝ → ℝ be a differentiable function with g(0) = 0, g ^ ′ (0)...

Let g:R→Rbeadifferentiablefunctionwithg: \mathbb{R} \to \mathbb{R} be a differentiable function withg\left(0\right) = 0, {g}^{′}\left(0\right) = 0 and {g}^{′}\left(1\right) \ne 0 .LetLetf\left(x\right) = \left\{{x∣x∣g(x),x≠00,x=0\begin{cases}\dfrac{x}{| x |}g\left(x\right), & x \ne 0 \\ 0, & x = 0\end{cases}\right) and h\left(x\right) = {e}^{| x |}forallfor allx \in \mathbb{R} .Let(foh)(x)Let \left(foh\right) ( x ) denote f(h(x))and(hof)(x)f\left(h\left(x\right)\right)and ( hof ) ( x )denote h(f(x))h\left(f\left(x\right)\right) .Then which of the following is (are) true?

Options

  1. Aff is differentiable at x=0x = 0
  2. Bhh is differentiable at x=0x = 0
  3. Cf0hf ₀ h is differentiable at x=0x = 0
  4. Dh0fh ₀ f is differentiable at x=0x = 0

Correct answer

A. ff is differentiable at x=0x = 0

Step-by-step solution

f\left(x\right) = \left\{\begin{cases}g\left(x\right) & x > 0 \\ 0 & x = 0 \\ - g\left(x\right) & x < 0\end{cases}\right) <br >LHD: \lim_{x \to {0}^{-}} \left(- {g}^{′}\left(x\right)\right) = - \lim_{x \to {0}^{-}} {g}^{′}\left(x\right) = - 0 = 0<br><br >RHD: \lim_{x \to {0}^{+}} {g}^{′}\left(x\right) = {g}^{′}\left({0}^{+}\right) = 0<br>Hence<br >Hencef\left(x\right)isdifferentiableatis differentiable atx = 0<br><br >h\left(x\right) = {e}^{\left|x\right|} = \left\{{exx≥0e−xx<0\begin{cases}{e}^{x} & x \ge 0 \\ {e}^{- x} & x < 0\end{cases}\right)
LHD:lim⁡x→0−(−e−x)=−1LHD: \lim_{x \to {0}^{-}} \left(- {e}^{- x}\right) = - 1
RHD:lim⁡x→0+(ex)=1RHD: \lim_{x \to {0}^{+}} \left({e}^{x}\right) = 1
Hence h(x)h\left(x\right) is not differentiable at x=0x = 0
f\left(h\left(x\right)\right) = \left\{\dfrac{{e}^{\left|x\right|}}{\left|{e}^{\left|x\right|}\right|}\rightg\left({e}^{\left|x\right|}\right)
f\left(h\left(x\right)\right) = \left\{\begin{cases}g\left({e}^{x}\right) & x \ge 0 \\ g\left({e}^{- x}\right) & x < 0\end{cases}\right) <br >LHD: \lim_{x \to {0}^{+}} {g}^{′}\left({e}^{- x}\right) \times - {e}^{- x} = {g}^{′}\left(1\right) \times - 1 = - {g}^{′}\left(1\right)<br><br >RHD: \lim_{x \to {0}^{+}} {g}^{′}\left({e}^{x}\right) \times{e}^{x} = {g}^{′}\left(1\right)<br>Hencefohisnon−differentiableat<br >Hence foh is non-differentiable atx = 0<br><br >h\left(f\left(x\right)\right) = \left\{{1x=0e∣x∣x∣g(x)∣x≠0\begin{cases}1 & x = 0 \\ {e}^{\left|\dfrac{x}{\left|x\right|}g\left(x\right)\right|} & x \ne 0\end{cases}\right)
= \left\{\begin{cases}1 & x = 0 \\ {e}^{\left|g\left(x\right)\right|} & x \ne 0\end{cases}\right) <br >LHD: \lim_{x \to {0}^{-}} {e}^{\left|g\left(x\right)\right|} \times \pm {g}^{′}\left(x\right) = {e}^{\left|g\left({0}^{-}\right)\right|} \times \pm \left({g}^{′}\left({0}^{-}\right)\right) = 0<br><br >RHD: \lim_{x \to {0}^{+}} {e}^{\left|g\left(x\right)\right|} \times \pm {g}^{′}\left(x\right) = {e}^{\left|g\left({0}^{+}\right)\right|} \times \pm {g}^{′}\left({0}^{+}\right) = 0
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