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Limits Continuity and Differentiability question

2020 · 3 Sep · Shift 1 · Q21
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  5. /2020 · 3 Sep · Shift 1 · Q21

Limits Continuity and Differentiability question

2020 · 3 Sep · Shift 1 · Q21

JEE MainMathematicsLimits Continuity and DifferentiabilityMCQ+4 / −1
Let [t] denote the greatest integer ≤\le≤ t. If for some λ∈\lambda \inλ∈ R - {1, 0}, lim⁡x→0∣1−x+∣x∣λ−x+[x]∣\mathop {\lim }\limits_{x \to 0} \left| {{{1 - x + \left| x \right|} \over {\lambda - x + \left[ x \right]}}} \right|x→0lim​​λ−x+[x]1−x+∣x∣​​ = L, then L is equal to :
  1. A
    1
  2. B
    2
  3. C
    0
  4. D
    12{1 \over 2}21​
View written solutionFree

Correct answer: B

  1. We need to evaluate L=lim⁡x→0∣1−x+∣x∣λ−x+[x]∣,L=\lim_{x\to 0}\left|\frac{1-x+|x|}{\lambda-x+[x]}\right|,L=limx→0​​λ−x+[x]1−x+∣x∣​​, where [x][x][x] is the greatest integer function and λ∈R∖{0,1}\lambda\in\mathbb R\setminus\{0,1\}λ∈R∖{0,1}.

  2. Since both ∣x∣|x|∣x∣ and [x][x][x] behave differently for x→0+x\to 0^+x→0+ and x→0−x\to 0^-x→0−, we check one-sided limits.


Case 1: x→0+x\to 0^+x→0+

For x>0x>0x>0 sufficiently small, ∣x∣=x,[x]=0.|x|=x, \qquad [x]=0.∣x∣=x,[x]=0. So, 1−x+∣x∣=1−x+x=1,1-x+|x|=1-x+x=1,1−x+∣x∣=1−x+x=1, and λ−x+[x]=λ−x.\lambda-x+[x]=\lambda-x.λ−x+[x]=λ−x. Hence ∣1−x+∣x∣λ−x+[x]∣=∣1λ−x∣.\left|\frac{1-x+|x|}{\lambda-x+[x]}\right|=\left|\frac{1}{\lambda-x}\right|.​λ−x+[x]1−x+∣x∣​​=​λ−x1​​. Therefore, lim⁡x→0+∣1−x+∣x∣λ−x+[x]∣=1∣λ∣.\lim_{x\to 0^+}\left|\frac{1-x+|x|}{\lambda-x+[x]}\right|=\frac{1}{|\lambda|}.limx→0+​​λ−x+[x]1−x+∣x∣​​=∣λ∣1​.


Case 2: x→0−x\to 0^-x→0−

For x<0x<0x<0 sufficiently close to 000, ∣x∣=−x,[x]=−1.|x|=-x, \qquad [x]=-1.∣x∣=−x,[x]=−1. So, 1−x+∣x∣=1−x−x=1−2x,1-x+|x|=1-x-x=1-2x,1−x+∣x∣=1−x−x=1−2x, and λ−x+[x]=λ−x−1.\lambda-x+[x]=\lambda-x-1.λ−x+[x]=λ−x−1. Thus, ∣1−x+∣x∣λ−x+[x]∣=∣1−2xλ−1−x∣.\left|\frac{1-x+|x|}{\lambda-x+[x]}\right|=\left|\frac{1-2x}{\lambda-1-x}\right|.​λ−x+[x]1−x+∣x∣​​=​λ−1−x1−2x​​. Taking limit as x→0−x\to 0^-x→0−, lim⁡x→0−∣1−x+∣x∣λ−x+[x]∣=∣1λ−1∣=1∣λ−1∣.\lim_{x\to 0^-}\left|\frac{1-x+|x|}{\lambda-x+[x]}\right|=\left|\frac{1}{\lambda-1}\right|=\frac{1}{|\lambda-1|}.limx→0−​​λ−x+[x]1−x+∣x∣​​=​λ−11​​=∣λ−1∣1​.


  1. For the two-sided limit to exist, the right-hand and left-hand limits must be equal: 1∣λ∣=1∣λ−1∣.\frac{1}{|\lambda|}=\frac{1}{|\lambda-1|}.∣λ∣1​=∣λ−1∣1​. So, ∣λ∣=∣λ−1∣.|\lambda|=|\lambda-1|.∣λ∣=∣λ−1∣.

  2. Solve this equation: λ2=(λ−1)2\lambda^2=(\lambda-1)^2λ2=(λ−1)2 λ2=λ2−2λ+1\lambda^2=\lambda^2-2\lambda+1λ2=λ2−2λ+1 2λ=12\lambda=12λ=1 λ=12.\lambda=\frac12.λ=21​. This satisfies the condition λ≠0,1\lambda\ne 0,1λ=0,1.

  3. Now compute the common limit: L=1∣λ∣=1∣12∣=2.L=\frac{1}{|\lambda|}=\frac{1}{\left|\frac12\right|}=2.L=∣λ∣1​=∣21​∣1​=2.

Thus, L=2.L=2.L=2.

  1. Comparing with the options, the correct option is B.
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