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JEE Advanced 2024 · Limits and Derivatives

Let SS be the set of all (α,β)∈R×R(\alpha, \beta) \in \mathbb{R} \times \mathbb{R} such that lim⁡x→∞sin⁡(x2)(log⁡ex)αsin⁡(1x2)xαβ(log⁡e(1+x))β=0\lim_{x \to \infty} \frac{\sin(x^{2})(\log_{e} x)^{\alpha} \sin\left(\frac{1}{x^{2}}\right)}{x^{\alpha\beta}(\log_{e}(1+x))^{\beta}} = 0 Then which of the following is (are) correct?

Answers: B. (−1,1)∈S(-1, 1) \in S · C. (1,−1)∈S(1, -1) \in S

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Written solution

Let S be the set of all (α, β) ∈ R × R such that lim_(x → ∞) (sin(x²)(logₑ x)^α sin(1/x²))/(x^(αβ)(logₑ(1+x))^β) = 0 Then which of the following is (are) correct?

S⊆R2: lim⁡x→∞sin⁡(x2) (ln⁡x)α sin⁡(1/x2)xαβ (ln⁡(1+x))β=0S \subseteq \mathbb{R}^2:\ \lim_{x\to\infty} \dfrac{\sin(x^2)\,(\ln x)^\alpha\,\sin(1/x^2)}{x^{\alpha\beta}\,(\ln(1+x))^\beta} = 0

four given pairs (α,β)\text{four given pairs } (\alpha,\beta)

Check: (−1,3), (−1,1), (1,−1), (1,−2) ∈S ?\text{Check: } (-1,3),\ (-1,1),\ (1,-1),\ (1,-2)\ \in S\ ?

sin⁡(1/x2)∼1x2,ln⁡(1+x)∼ln⁡x\sin(1/x^2) \sim \dfrac{1}{x^2},\qquad \ln(1+x) \sim \ln x

∣sin⁡(x2)∣≤1(bounded, oscillating)|\sin(x^2)| \le 1 \quad (\text{bounded, oscillating})

(ln⁡x)α⋅1x2xαβ(ln⁡x)β=(ln⁡x)α−βx2+αβ\dfrac{(\ln x)^\alpha \cdot \frac{1}{x^2}}{x^{\alpha\beta}(\ln x)^\beta} = \dfrac{(\ln x)^{\alpha-\beta}}{x^{2+\alpha\beta}}

expression∼sin⁡(x2)⋅(ln⁡x)α−βx2+αβ\text{expression} \sim \sin(x^2)\cdot\dfrac{(\ln x)^{\alpha-\beta}}{x^{2+\alpha\beta}}

Since sin⁡(x2) oscillates: limit =0  ⟺  A(x)→0\text{Since } \sin(x^2) \text{ oscillates: limit } = 0 \iff A(x) \to 0

A(x)=(ln⁡x)α−βx2+αβA(x) = \dfrac{(\ln x)^{\alpha-\beta}}{x^{2+\alpha\beta}}

2+αβ>0⇒A(x)→02 + \alpha\beta > 0 \Rightarrow A(x) \to 0

2+αβ<0⇒A(x)→∞2 + \alpha\beta < 0 \Rightarrow A(x) \to \infty

2+αβ=0⇒A(x)=(ln⁡x)α−β→0  ⟺  α<β2 + \alpha\beta = 0 \Rightarrow A(x) = (\ln x)^{\alpha-\beta} \to 0 \iff \alpha < \beta

S={(α,β):αβ>−2}∪{(α,β):αβ=−2, α<β}S = \{(\alpha,\beta): \alpha\beta > -2\} \cup \{(\alpha,\beta): \alpha\beta = -2,\ \alpha < \beta\}

(1) (−1,3): αβ=−3<−2 ×(1)\ (-1,3):\ \alpha\beta = -3 < -2 \ \times

(2) (−1,1): αβ=−1>−2 ✓(2)\ (-1,1):\ \alpha\beta = -1 > -2 \ \checkmark

(3) (1,−1): αβ=−1>−2 ✓(3)\ (1,-1):\ \alpha\beta = -1 > -2 \ \checkmark

(4) (1,−2): αβ=−2, α>β ×(4)\ (1,-2):\ \alpha\beta = -2,\ \alpha > \beta \ \times

Correct options: (2) and (3)\boxed{\text{Correct options: } (2)\ \text{and}\ (3)}

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