arthham

JEE Advanced 2015 · Integrals

If α=∫01(e9x+3tan⁡−1x)(12+9x21+x2)dx\alpha = \int_{0}^{1} \left(e^{9x+3\tan^{-1}x}\right) \left(\frac{12+9x^{2}}{1+x^{2}}\right) dx where tan⁡−1x\tan^{-1}x takes only principal values, then the value of (log⁡e∣1+α∣−3π4)\left(\log_e |1+\alpha| - \frac{3\pi}{4}\right) is

Answer: 99

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Written solution

If α = ∫₀¹ (e^(9x+3tan⁻¹x)) ((12+9x²)/(1+x²)) dx where tan⁻¹x takes only principal values, then the value of (logₑ |1+α| - 3π/4) is α=∫01e9x+3tan⁡−1x(12+9x21+x2)dx\alpha = \int_{0}^{1} e^{9x+3\tan^{-1}x}\left(\frac{12+9x^{2}}{1+x^{2}}\right)dx Find α then log⁡e∣1+α∣−3π4\text{Find } \alpha \ \text{then } \log_e|1+\alpha| - \frac{3\pi}{4} 1. g(x)=9x+3tan⁡−1x⇒g′(x)1.\ g(x) = 9x+3\tan^{-1}x \Rightarrow g'(x) 2. α=∫egg′ dx=[eg]2.\ \alpha = \int e^{g}g'\,dx = \left[e^{g}\right] 3. use limits ⇒log⁡e∣1+α∣−3π43.\ \text{use limits } \Rightarrow \log_e|1+\alpha| - \frac{3\pi}{4} ddx(9x+3tan⁡−1x)=9+31+x2\frac{d}{dx}\left(9x + 3\tan^{-1}x\right) = 9 + \frac{3}{1+x^2} =9(1+x2)+31+x2=12+9x21+x2= \frac{9(1+x^2)+3}{1+x^2} = \frac{12+9x^2}{1+x^2} α=∫01eg(x)g′(x) dx,g(x)=9x+3tan⁡−1x\alpha = \int_0^1 e^{g(x)} g'(x)\,dx,\quad g(x)=9x+3\tan^{-1}x =[eg(x)]01=eg(1)−eg(0)= \left[e^{g(x)}\right]_0^1 = e^{g(1)} - e^{g(0)} g(1)=9+3⋅π4=9+3π4,g(0)=0g(1) = 9 + 3\cdot\frac{\pi}{4} = 9 + \frac{3\pi}{4},\qquad g(0)=0 α=e9+3π/4−1\alpha = e^{9+3\pi/4} - 1 1+α=e9+3π/4⇒log⁡e∣1+α∣=9+3π41+\alpha = e^{9+3\pi/4} \Rightarrow \log_e|1+\alpha| = 9 + \frac{3\pi}{4} log⁡e∣1+α∣−3π4=9\log_e|1+\alpha| - \frac{3\pi}{4} = 9

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