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Limits Continuity and Differentiability question

2024 · Shift 2 · Q22
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  5. /2024 · Shift 2 · Q22

Limits Continuity and Differentiability question

2024 · Shift 2 · Q22

JEE AdvancedMathematicsLimits Continuity and DifferentiabilityMultiple correct+4 / −2
Let SSS be the set of all (α,β)∈R×R(\alpha, \beta) \in \mathbb{R} \times \mathbb{R}(α,β)∈R×R such that lim⁡x→∞sin⁡(x2)(log⁡ex)αsin⁡(1x2)xαβ(log⁡e(1+x))β=0.\lim\limits_{x \rightarrow \infty} \frac{\sin \left(x^2\right)\left(\log _e x\right)^\alpha \sin \left(\frac{1}{x^2}\right)}{x^{\alpha \beta}\left(\log _e(1+x)\right)^\beta}=0 .x→∞lim​xαβ(loge​(1+x))βsin(x2)(loge​x)αsin(x21​)​=0. Then which of the following is (are) correct?
  1. A
    (−1,3)∈S(-1,3) \in S(−1,3)∈S
  2. B
    (−1,1)∈S(-1,1) \in S(−1,1)∈S
  3. C
    (1,−1)∈S(1,-1) \in S(1,−1)∈S
  4. D
    (1,−2)∈S(1,-2) \in S(1,−2)∈S
View written solutionFree

Correct answer: B, C

Step-by-step Derivations:

  1. Analyze the Limit Expression

    Let the given limit be LLL. The expression inside the limit is: f(x)=sin⁡(x2)(log⁡ex)αsin⁡(1x2)xαβ(log⁡e(1+x))βf(x) = \frac{\sin \left(x^2\right)\left(\log _e x\right)^\alpha \sin \left(\frac{1}{x^2}\right)}{x^{\alpha \beta}\left(\log _e(1+x)\right)^\beta}f(x)=xαβ(loge​(1+x))βsin(x2)(loge​x)αsin(x21​)​ We need to find the set SSS of all pairs (α,β)(\alpha, \beta)(α,β) for which lim⁡x→∞f(x)=0\lim_{x \to \infty} f(x) = 0limx→∞​f(x)=0.

  2. Use Asymptotic Equivalents for x→∞x \rightarrow \inftyx→∞

    For large values of xxx, we can use the following standard limits and approximations:

    • As x→∞x \rightarrow \inftyx→∞, 1x2→0\frac{1}{x^2} \rightarrow 0x21​→0. Therefore, sin⁡(1x2)∼1x2\sin\left(\frac{1}{x^2}\right) \sim \frac{1}{x^2}sin(x21​)∼x21​. This is because lim⁡u→0sin⁡uu=1\lim_{u \to 0} \frac{\sin u}{u} = 1limu→0​usinu​=1.
    • As x→∞x \rightarrow \inftyx→∞, log⁡e(1+x)=log⁡e(x(1+1/x))=log⁡ex+log⁡e(1+1/x)\log_e(1+x) = \log_e(x(1+1/x)) = \log_e x + \log_e(1+1/x)loge​(1+x)=loge​(x(1+1/x))=loge​x+loge​(1+1/x). Since log⁡e(1+1/x)→0\log_e(1+1/x) \to 0loge​(1+1/x)→0, we have log⁡e(1+x)∼log⁡ex\log_e(1+x) \sim \log_e xloge​(1+x)∼loge​x. More formally, lim⁡x→∞log⁡e(1+x)log⁡ex=1\lim_{x \to \infty} \frac{\log_e(1+x)}{\log_e x} = 1limx→∞​loge​xloge​(1+x)​=1.
  3. Simplify the Expression

    Based on these equivalents, the behavior of f(x)f(x)f(x) for large xxx is similar to the behavior of the following expression: g(x)=sin⁡(x2)(log⁡ex)α(1x2)xαβ(log⁡ex)βg(x) = \frac{\sin \left(x^2\right)\left(\log _e x\right)^\alpha \left(\frac{1}{x^2}\right)}{x^{\alpha \beta}\left(\log _e x\right)^\beta}g(x)=xαβ(loge​x)βsin(x2)(loge​x)α(x21​)​ Simplifying g(x)g(x)g(x): g(x)=sin⁡(x2)⋅(log⁡ex)α−βx2+αβg(x) = \sin(x^2) \cdot \frac{(\log_e x)^{\alpha - \beta}}{x^{2+\alpha\beta}}g(x)=sin(x2)⋅x2+αβ(loge​x)α−β​ The limit of f(x)f(x)f(x) is 0 if and only if the limit of g(x)g(x)g(x) is 0, because lim⁡x→∞f(x)g(x)=lim⁡x→∞sin⁡(1/x2)1/x2⋅(log⁡exlog⁡e(1+x))β=1⋅1β=1\lim_{x \to \infty} \frac{f(x)}{g(x)} = \lim_{x \to \infty} \frac{\sin(1/x^2)}{1/x^2} \cdot \left(\frac{\log_e x}{\log_e(1+x)}\right)^\beta = 1 \cdot 1^\beta = 1limx→∞​g(x)f(x)​=limx→∞​1/x2sin(1/x2)​⋅(loge​(1+x)loge​x​)β=1⋅1β=1.

  4. Determine the Conditions for the Limit to be Zero

    We need to find when L=lim⁡x→∞sin⁡(x2)⋅(log⁡ex)α−βx2+αβ=0L = \lim_{x \rightarrow \infty} \sin(x^2) \cdot \frac{(\log_e x)^{\alpha-\beta}}{x^{2+\alpha \beta}} = 0L=limx→∞​sin(x2)⋅x2+αβ(loge​x)α−β​=0. The term sin⁡(x2)\sin(x^2)sin(x2) is a bounded function that oscillates between -1 and 1. For the overall limit to be zero, the magnitude of the other part must tend to zero. This is a direct application of the Squeeze Theorem. So, the condition is: lim⁡x→∞(log⁡ex)α−βx2+αβ=0\lim_{x \rightarrow \infty} \frac{(\log_e x)^{\alpha-\beta}}{x^{2+\alpha \beta}} = 0limx→∞​x2+αβ(loge​x)α−β​=0 We analyze this limit based on the power of xxx in the denominator, which is 2+αβ2+\alpha\beta2+αβ.

    • Case 1: 2+αβ>02 + \alpha\beta > 02+αβ>0 In this case, the power of xxx in the denominator is positive. Since polynomial functions grow faster than logarithmic functions, the limit will be 0, regardless of the value of the exponent α−β\alpha-\betaα−β. So, if 2+αβ>02 + \alpha\beta > 02+αβ>0, then (α,β)∈S(\alpha, \beta) \in S(α,β)∈S.

    • Case 2: 2+αβ<02 + \alpha\beta < 02+αβ<0 Let 2+αβ=−k2 + \alpha\beta = -k2+αβ=−k where k>0k > 0k>0. The expression becomes lim⁡x→∞xk(log⁡ex)α−β\lim_{x \rightarrow \infty} x^k (\log_e x)^{\alpha-\beta}limx→∞​xk(loge​x)α−β. This limit is ∞\infty∞, not 0. So these pairs are not in SSS.

    • Case 3: 2+αβ=02 + \alpha\beta = 02+αβ=0 The expression becomes lim⁡x→∞(log⁡ex)α−β\lim_{x \rightarrow \infty} (\log_e x)^{\alpha-\beta}limx→∞​(loge​x)α−β. For this limit to be 0, the exponent must be negative. So, we need α−β<0\alpha - \beta < 0α−β<0. If α−β≥0\alpha - \beta \ge 0α−β≥0, the limit is ∞\infty∞ or 1, and the original limit LLL would not be 0 (it would not exist).

    Summary of Conditions: A pair (α,β)(\alpha, \beta)(α,β) is in the set SSS if and only if: (i) 2+αβ>02 + \alpha\beta > 02+αβ>0 OR (ii) 2+αβ=02 + \alpha\beta = 02+αβ=0 AND α−β<0\alpha - \beta < 0α−β<0.

  5. Evaluate Each Option

    • A: (α,β)=(−1,3)(\alpha, \beta) = (-1, 3)(α,β)=(−1,3) 2+αβ=2+(−1)(3)=2−3=−12 + \alpha\beta = 2 + (-1)(3) = 2 - 3 = -12+αβ=2+(−1)(3)=2−3=−1. Since −1<0-1 < 0−1<0, condition (i) is not met. Condition (ii) is also not met. Thus, (−1,3)∉S(-1,3) \notin S(−1,3)∈/S.

    • B: (α,β)=(−1,1)(\alpha, \beta) = (-1, 1)(α,β)=(−1,1) 2+αβ=2+(−1)(1)=2−1=12 + \alpha\beta = 2 + (-1)(1) = 2 - 1 = 12+αβ=2+(−1)(1)=2−1=1. Since 1>01 > 01>0, condition (i) is met. Thus, (−1,1)∈S(-1,1) \in S(−1,1)∈S.

    • C: (α,β)=(1,−1)(\alpha, \beta) = (1, -1)(α,β)=(1,−1) 2+αβ=2+(1)(−1)=2−1=12 + \alpha\beta = 2 + (1)(-1) = 2 - 1 = 12+αβ=2+(1)(−1)=2−1=1. Since 1>01 > 01>0, condition (i) is met. Thus, (1,−1)∈S(1,-1) \in S(1,−1)∈S.

    • D: (α,β)=(1,−2)(\alpha, \beta) = (1, -2)(α,β)=(1,−2) 2+αβ=2+(1)(−2)=2−2=02 + \alpha\beta = 2 + (1)(-2) = 2 - 2 = 02+αβ=2+(1)(−2)=2−2=0. Condition (i) is not met. We check condition (ii). We have 2+αβ=02 + \alpha\beta = 02+αβ=0. Now check the second part of condition (ii): α−β=1−(−2)=3\alpha - \beta = 1 - (-2) = 3α−β=1−(−2)=3. Since 333 is not less than 0, the condition α−β<0\alpha - \beta < 0α−β<0 is not met. Thus, (1,−2)∉S(1,-2) \notin S(1,−2)∈/S.

Conclusion:

The pairs (α,β)(\alpha, \beta)(α,β) that belong to the set SSS are (−1,1)(-1,1)(−1,1) and (1,−1)(1,-1)(1,−1). Therefore, options B and C are correct.

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