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Limits Continuity and Differentiability question

2024 · 31 Jan · Shift 2 · Q47
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  5. /2024 · 31 Jan · Shift 2 · Q47

Limits Continuity and Differentiability question

2024 · 31 Jan · Shift 2 · Q47

JEE MainMathematicsLimits Continuity and DifferentiabilityMCQ+4 / −1
Consider the function f:(0,∞)→Rf:(0, \infty) \rightarrow \mathbb{R}f:(0,∞)→R defined by f(x)=e−∣log⁡ex∣f(x)=e^{-\left|\log _e x\right|}f(x)=e−∣loge​x∣. If mmm and nnn be respectively the number of points at which fff is not continuous and fff is not differentiable, then m+nm+nm+n is
  1. A
    0
  2. B
    1
  3. C
    2
  4. D
    3
View written solutionFree

Correct answer: B

  1. Given function

    f(x)=e−∣"logex∣, x∈(0,∞).f(x)=e^{-|"log_e x|}, \, x\in(0,\infty).f(x)=e−∣"loge​x∣,x∈(0,∞).

    Since log⁡ex=ln⁡x\log_e x=\ln xloge​x=lnx, write

\ln x|}.$$

  1. Remove the absolute value by cases

    We know:

    • If x≥1x\ge 1x≥1, then ln⁡x≥0\ln x\ge 0lnx≥0, so ∣ln⁡x∣=ln⁡x|\ln x|=\ln x∣lnx∣=lnx.
    • If 0<x<10<x<10<x<1, then ln⁡x<0\ln x<0lnx<0, so ∣ln⁡x∣=−ln⁡x|\ln x|=-\ln x∣lnx∣=−lnx.

    Therefore,

    \begin{cases} e^{-(-\ln x)}=e^{\ln x}=x, & 0<x<1,\\[4pt] e^{-\ln x}=\dfrac{1}{x}, & x\ge 1. \end{cases}$$ So, $$f(x)= \begin{cases} x, & 0<x<1,\\ \dfrac{1}{x}, & x\ge 1. \end{cases}$$
  2. Check continuity

    On (0,1)(0,1)(0,1), f(x)=xf(x)=xf(x)=x, which is continuous.

    On (1,∞)(1,\infty)(1,∞), f(x)=1xf(x)=\dfrac{1}{x}f(x)=x1​, which is continuous.

    The only possible issue is at x=1x=1x=1.

    • Left limit: lim⁡x→1−f(x)=lim⁡x→1−x=1.\lim_{x\to 1^-} f(x)=\lim_{x\to 1^-} x=1.limx→1−​f(x)=limx→1−​x=1.
    • Right limit: lim⁡x→1+f(x)=lim⁡x→1+1x=1.\lim_{x\to 1^+} f(x)=\lim_{x\to 1^+} \frac{1}{x}=1.limx→1+​f(x)=limx→1+​x1​=1.
    • Function value:

\ln 1|}=e^0=1.$$

Hence fff is continuous at x=1x=1x=1 as well.

Therefore, the number of points where fff is not continuous is m=0.m=0.m=0.

  1. Check differentiability

    On (0,1)(0,1)(0,1), f(x)=x  ⟹  f′(x)=1.f(x)=x \implies f'(x)=1.f(x)=x⟹f′(x)=1.

    On (1,∞)(1,\infty)(1,∞), f(x)=1x  ⟹  f′(x)=−1x2.f(x)=\frac{1}{x} \implies f'(x)=-\frac{1}{x^2}.f(x)=x1​⟹f′(x)=−x21​.

    Again, only x=1x=1x=1 needs checking.

    • Left derivative at x=1x=1x=1: f−′(1)=1.f'_-(1)=1.f−′​(1)=1.
    • Right derivative at x=1x=1x=1: f+′(1)=−112=−1.f'_+(1)=-\frac{1}{1^2}=-1.f+′​(1)=−121​=−1.

    Since f−′(1)≠f+′(1),f'_-(1)\ne f'_+(1),f−′​(1)=f+′​(1), fff is not differentiable at x=1x=1x=1.

    Therefore, the number of points where fff is not differentiable is n=1.n=1.n=1.

  2. Compute m+nm+nm+n

    m+n=0+1=1.m+n=0+1=1.m+n=0+1=1.

  3. Match with options

    Option B: 1 is correct.

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