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BITSAT 2018
Exam
BITSAT
Year
2018
Subject
Mathematics
Chapter
Continuity and Differentiability

BITSAT 2018 · Mathematics · Continuity and Differentiability

Let f(x) = \ array ll x^ p sin(1)/(x), x ≠ 0 array cases 0, & x = 0...

Let f\left(x\right) = \left\{xpsin1x,x≠0\begin{array}{ll}x^{p}sin\dfrac{1}{x}, x \ne 0\end{array}\begin{cases}0, & x = 0\end{cases}\right) then f \left(x\right)iscontinuousbutnotdifferentiableatis continuous but not differentiable atx = 0, if

Options

  1. Ap<0p < 0
  2. Bp=0p = 0
  3. C0<p≤10 < p \le 1
  4. Dp≥1p \ge 1

Correct answer

C. 0<p≤10 < p \le 1

Step-by-step solution

We have,

ff\left(x\right) = \left\{xpsin1x,x≠0\begin{array}{ll}{x}^{p}sin\dfrac{1}{x}, x \ne 0\end{array} {0,x=0\begin{cases}0, & x = 0\end{cases}\right)

For f(x)f\left(x\right) to be continuous at x=0x = 0

lim⁡x→0f(x)=f(0)\lim_{x \to 0}f\left(x\right) = f\left(0\right)

⇒lim⁡x→0(xpsin1x)=0⇒ \lim_{x \to 0}\left({x}^{p}sin\dfrac{1}{x}\right) = 0

⇒lim⁡x→0(xp)×lim⁡x→0(sin1x)=0⇒ \lim_{x \to 0}\left({x}^{p}\right) \times \lim_{x \to 0}\left(sin\dfrac{1}{x}\right) = 0

⇒lim⁡x→0(xp)×(value between−1to1)=0⇒ \lim_{x \to 0}\left({x}^{p}\right) \times \left(\text{value between} - 1\text{to}1\right) = 0

∴p>0∴ p > 0

Now,

RHD at RHD at(x=0)=lim⁡h→0[hpsin1h−0h]=lim⁡h→0[hp−1sin1h]\text{RHD at}\left(x = 0\right) = \lim_{h \to 0}\left[\dfrac{{h}^{p}sin\dfrac{1}{h} - 0}{h}\right] = \lim_{h \to 0}\left[{h}^{p - 1}sin\dfrac{1}{h}\right]

LHD at(x=0)=lim⁡h→0[(−h)psin⁡(−1h)−0−h]=lim⁡h→0[(−1)p(−h)p−1sin⁡(1h)]\text{LHD at}\left(x = 0\right) = \lim_{h \to 0}\left[\dfrac{{\left(- h\right)}^{p}\sin \left(- \dfrac{1}{h}\right) - 0}{- h}\right] = \lim_{h \to 0}\left[{\left(- 1\right)}^{p}{\left(- h\right)}^{p - 1}\sin \left(\dfrac{1}{h}\right)\right]

Since, f(x)f\left(x\right) is not differentiable at x=0x = 0, therefore

p≤1p \le 1

Hence, 0<p≤10 < p \le 1.

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