arthham

JEE Advanced 2022 · Integrals

Consider the equation ∫1e(log⁡ex)1/2x(a−(log⁡ex)3/2)2 dx=1,a∈(−∞,0)∪(1,∞).\displaystyle\int_{1}^{e} \frac{(\log_e x)^{1/2}}{x\left(a-(\log_e x)^{3/2}\right)^{2}}\,dx = 1, \quad a \in (-\infty,0)\cup(1,\infty).

Answers: C. An irrational number aa satisfies the above equation · D. More than one aa satisfy the above equation

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Written solution

Consider the equation ∫₁ᵉ ((logₑ x)^(1/2))/(x(a-(logₑ x)^(3/2))²) dx = 1, a ∈ (-∞,0)∪(1,∞). ∫1e(log⁡ex)1/2x(a−(log⁡ex)3/2)2 dx=1\int_1^e \dfrac{(\log_e x)^{1/2}}{x\left(a-(\log_e x)^{3/2}\right)^{2}}\,dx = 1 a∈(−∞,0)∪(1,∞)a \in (-\infty,0)\cup(1,\infty) To find: which statements about a are correct\text{To find: which statements about } a \text{ are correct} Plan: put u=(log⁡ex)3/2, integrate, then solve for a\text{Plan: put } u=(\log_e x)^{3/2},\ \text{integrate, then solve for } a u=(log⁡ex)3/2⇒du=32(log⁡ex)1/2dxxu=(\log_e x)^{3/2} \Rightarrow du = \dfrac{3}{2}(\log_e x)^{1/2}\dfrac{dx}{x} (log⁡ex)1/2x dx=23 du;x=1→u=0,  x=e→u=1\dfrac{(\log_e x)^{1/2}}{x}\,dx = \dfrac{2}{3}\,du;\qquad x=1\to u=0,\ \ x=e\to u=1 I=23∫01du(a−u)2I = \dfrac{2}{3}\int_0^1 \dfrac{du}{(a-u)^2} ∫du(a−u)2=1a−u\int \dfrac{du}{(a-u)^2} = \dfrac{1}{a-u} I=23[1a−1−1a]=23⋅1a(a−1)I = \dfrac{2}{3}\left[\dfrac{1}{a-1} - \dfrac{1}{a}\right] = \dfrac{2}{3}\cdot\dfrac{1}{a(a-1)} 23⋅1a(a−1)=1⇒a(a−1)=23\dfrac{2}{3}\cdot\dfrac{1}{a(a-1)} = 1 \Rightarrow a(a-1)=\dfrac{2}{3} 3a2−3a−2=03a^2 - 3a - 2 = 0 a=3±9+246=3±336a = \dfrac{3\pm\sqrt{9+24}}{6} = \dfrac{3\pm\sqrt{33}}{6} a1=3+336≈1.46>1 ✓,a2=3−336≈−0.46<0 ✓a_1=\dfrac{3+\sqrt{33}}{6}\approx 1.46>1\ \checkmark,\qquad a_2=\dfrac{3-\sqrt{33}}{6}\approx -0.46<0\ \checkmark (1) FALSE(2) FALSE(3) TRUE(4) TRUE(1)\ \text{FALSE}\quad (2)\ \text{FALSE}\quad (3)\ \text{TRUE}\quad (4)\ \text{TRUE} (3) and (4)\boxed{(3)\ \text{and}\ (4)}

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