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Limits Continuity and Differentiability question

2002 · Shift 0 · Q68
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  5. /2002 · Shift 0 · Q68

Limits Continuity and Differentiability question

2002 · Shift 0 · Q68

JEE MainMathematicsLimits Continuity and DifferentiabilityMCQ+4 / −1
lim⁡x→0log⁡xn−[x][x]\mathop {\lim }\limits_{x \to 0} {{\log {x^n} - \left[ x \right]} \over {\left[ x \right]}}x→0lim​[x]logxn−[x]​, n∈Nn \in Nn∈N, ( [x] denotes the greatest integer less than or equal to x )
  1. A
    has value −1-1−1
  2. B
    has value 000
  3. C
    has value 111
  4. D
    does not exist
View written solutionFree

Correct answer: D

  1. Interpret the expression carefully

We need to evaluate

lim⁡x→0log⁡(xn)−[x][x],n∈N,\lim_{x\to 0} \frac{\log(x^n)-[x]}{[x]}, \qquad n\in \mathbb N,x→0lim​[x]log(xn)−[x]​,n∈N,

where [x][x][x] is the greatest integer function.

For real logarithm, log⁡(xn)\log(x^n)log(xn) is defined near x=0x=0x=0 only when xn>0x^n>0xn>0.

  • If nnn is even, then for x≠0x\neq 0x=0, xn>0x^n>0xn>0, so both sides are allowed.
  • If nnn is odd, then for real log, we need x>0x>0x>0, so only the right-hand side is relevant.

In either case, to test existence of the limit, it is enough to inspect values of [x][x][x] near 000.


  1. Behavior of [x][x][x] near x=0x=0x=0

For x→0+x\to 0^+x→0+), we have 0<x<10<x<10<x<1, hence

[x]=0.[x]=0.[x]=0.

Then the given expression becomes

log⁡(xn)−00,\frac{\log(x^n)-0}{0},0log(xn)−0​,

which is not defined. So there are no domain points arbitrarily close to 000 from the right where the expression is defined.

For x→0−x\to 0^-x→0−, we have −1<x<0-1<x<0−1<x<0, hence

[x]=−1.[x]=-1.[x]=−1.

Then

log⁡(xn)−[x][x]=log⁡(xn)+1−1=−log⁡(xn)−1.\frac{\log(x^n)-[x]}{[x]} =\frac{\log(x^n)+1}{-1} =-\log(x^n)-1.[x]log(xn)−[x]​=−1log(xn)+1​=−log(xn)−1.

Now:

  • if nnn is even, xn>0x^n>0xn>0 and as x→0−x\to 0^-x→0−, log⁡(xn)=nlog⁡∣x∣→−∞,\log(x^n)=n\log|x|\to -\infty,log(xn)=nlog∣x∣→−∞, so −log⁡(xn)−1→+∞.-\log(x^n)-1\to +\infty.−log(xn)−1→+∞.
  • if nnn is odd, log⁡(xn)\log(x^n)log(xn) is not real-defined for x<0x<0x<0.

So in no case do we get a finite two-sided limit.


  1. Conclusion

The expression is not defined on the right side near 000 because [x]=0[x]=0[x]=0 for 0<x<10<x<10<x<1, making the denominator zero. Hence the required limit does not exist.

Therefore, the correct option is

D: does not exist.\boxed{\text{D: does not exist}}.D: does not exist​.
  1. Comparison with stored answer

Stored correct answer: D.

My derived answer is also D, so they agree.

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