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Limits Continuity and Differentiability question

2008 · Shift 1 · Q44
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  5. /2008 · Shift 1 · Q44

Limits Continuity and Differentiability question

2008 · Shift 1 · Q44

JEE AdvancedMathematicsLimits Continuity and DifferentiabilityMCQ+3 / −1
Let g(x)=(x−1)nlog⁡cos⁡m(x−1);00g(x) = {{{{(x - 1)}^n}} \over {\log {{\cos }^m}(x - 1)}};0 0g(x)=logcosm(x−1)(x−1)n​;00, and let ppp be the left hand derivative of ∣x−1∣|x - 1|∣x−1∣ at x=1x = 1x=1. If lim⁡x→1+g(x)=p\mathop {\lim }\limits_{x \to {1^ + }} g(x) = px→1+lim​g(x)=p, then
  1. A
    n=1,m=1n = 1,m = 1n=1,m=1
  2. B
    n=1,m=−1n = 1,m = - 1n=1,m=−1
  3. C
    n=2,m=2n = 2,m = 2n=2,m=2
  4. D
    n>2,m=nn \gt 2,m = nn>2,m=n
View written solutionFree

Correct answer: C

  1. Interpret the given function

    The expression is g(x)=(x−1)nlog⁡(cos⁡m(x−1)).g(x)=\frac{(x-1)^n}{\log\left(\cos^m(x-1)\right)}.g(x)=log(cosm(x−1))(x−1)n​.

    We are also given that p=left hand derivative of ∣x−1∣ at x=1.p=\text{left hand derivative of } |x-1| \text{ at } x=1.p=left hand derivative of ∣x−1∣ at x=1.

  2. Find }p$

    Let f(x)=∣x−1∣.f(x)=|x-1|.f(x)=∣x−1∣.

    For x<1x<1x<1, ∣x−1∣=1−x,|x-1|=1-x,∣x−1∣=1−x, so f′(x)=−1(x<1).f'(x)=-1 \quad (x<1).f′(x)=−1(x<1).

    Hence the left hand derivative at x=1x=1x=1 is p=−1.p=-1.p=−1.

  3. Evaluate the limit of }g(x)asasasx\to 1^+$

    Put t=x−1.t=x-1.t=x−1. Then as x→1+x\to 1^+x→1+, we have t→0+t\to 0^+t→0+.

    So g(x)=tnlog⁡(cos⁡mt).g(x)=\frac{t^n}{\log(\cos^m t)}.g(x)=log(cosmt)tn​.

    Using logarithm properties, log⁡(cos⁡mt)=mlog⁡(cos⁡t).\log(\cos^m t)=m\log(\cos t).log(cosmt)=mlog(cost).

    Therefore g(x)=tnmlog⁡(cos⁡t).g(x)=\frac{t^n}{m\log(\cos t)}.g(x)=mlog(cost)tn​.

  4. Use the standard expansion of }\log(\cos t)nearnearneart=0$

    We know cos⁡t=1−t22+o(t2).\cos t = 1-\frac{t^2}{2}+o(t^2).cost=1−2t2​+o(t2).

    Hence log⁡(cos⁡t)∼−t22as t→0.\log(\cos t) \sim -\frac{t^2}{2} \quad \text{as } t\to 0.log(cost)∼−2t2​as t→0.

    So mlog⁡(cos⁡t)∼−mt22.m\log(\cos t) \sim -\frac{m t^2}{2}.mlog(cost)∼−2mt2​.

    Thus g(x)=tnmlog⁡(cos⁡t)∼tn−mt22=−2mtn−2.g(x)=\frac{t^n}{m\log(\cos t)} \sim \frac{t^n}{-\frac{m t^2}{2}} = -\frac{2}{m}t^{n-2}.g(x)=mlog(cost)tn​∼−2mt2​tn​=−m2​tn−2.

  5. Match the limit with }p=-1$

    We need lim⁡x→1+g(x)=−1.\lim_{x\to 1^+} g(x)= -1.limx→1+​g(x)=−1.

    That is, lim⁡t→0+−2mtn−2=−1.\lim_{t\to 0^+} -\frac{2}{m}t^{n-2}=-1.limt→0+​−m2​tn−2=−1.

    Now analyze cases:

    • If n>2n>2n>2, then tn−2→0t^{n-2}\to 0tn−2→0, so limit =0=0=0, not −1-1−1.
    • If n<2n<2n<2, then tn−2→∞t^{n-2}\to \inftytn−2→∞ or undefined behavior, not finite −1-1−1.
    • Therefore, we must have n=2.n=2.n=2.

    Then g(x)→−2m.g(x) \to -\frac{2}{m}.g(x)→−m2​.

    For this to equal −1-1−1, −2m=−1  ⟹  m=2.-\frac{2}{m}=-1 \implies m=2.−m2​=−1⟹m=2.

  6. Check options

    • A: n=1,m=1n=1,m=1n=1,m=1 gives divergent behavior, not correct.
    • B: n=1,m=−1n=1,m=-1n=1,m=−1 gives divergent behavior, not correct.
    • C: n=2,m=2n=2,m=2n=2,m=2 gives limit −1-1−1, correct.
    • D: n>2,m=nn>2,m=nn>2,m=n gives limit 000, not correct.
  7. Conclusion

    The correct option is C.\boxed{\text{C}}.C​.

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