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NDA 2017
Exam
NDA
Year
2017
Subject
Mathematics
Chapter
Indefinite Integration

NDA 2017 · Mathematics · Indefinite Integration

∫(In x)^ -1 d x-∫(In x)^ -2 d x is equal to

∫(In⁡x)−1dx−∫(In⁡x)−2dx\int(\operatorname{In} x)^{-1} d x-\int(\operatorname{In} x)^{-2} d x is equal to

Options

  1. Ax(In⁡x)−1+cx(\operatorname{In} x)^{-1}+c
  2. Bx(In⁡x)−2+cx(\operatorname{In} x)^{-2}+c
  3. Cx(In⁡x)+cx(\operatorname{In} x)+c
  4. Dx(In⁡x)2+cx(\operatorname{In} x)^2+c

Correct answer

A. x(In⁡x)−1+cx(\operatorname{In} x)^{-1}+c

Step-by-step solution

∫(ln⁡x)−1dx−∫(ln⁡x)−2dx=∫[1ln⁡x−1(ln⁡x)2]dx\begin{aligned} & \int(\ln x)^{-1} d x-\int(\ln x)^{-2} d x \\ & =\int\left[\frac{1}{\ln x}-\frac{1}{(\ln x)^2}\right] d x\end{aligned} Put ℓnx=t⇒x=etdx=et.dt.\begin{aligned} & \text{ Put } \ell \mathrm{n} \mathrm{x}=\mathrm{t} \Rightarrow \mathrm{x}=\mathrm{e}^{\mathrm{t}} \\ & \mathrm{dx}=\mathrm{e}^{\mathrm{t}} . \mathrm{dt} .\end{aligned}∴∫[1ln⁡x−1(ln⁡x)2]dx=∫(1t−1t2)⋅et⋅dt=∫et⋅(1t−1t2)⋅dt\begin{aligned} & \therefore \int\left[\frac{1}{\ln \mathrm{x}}-\frac{1}{(\ln \mathrm{x})^2}\right] \mathrm{dx}=\int\left(\frac{1}{\mathrm{t}}-\frac{1}{\mathrm{t}^2}\right) \cdot \mathrm{e}^{\mathrm{t}} \cdot \mathrm{dt} \\ & =\int \mathrm{e}^{\mathrm{t}} \cdot\left(\frac{1}{\mathrm{t}}-\frac{1}{\mathrm{t}^2}\right) \cdot \mathrm{dt}\end{aligned}=∫et⋅(1t−1t2)⋅dt=ett+c=xln⁡x+c\begin{aligned} & =\int \mathrm{e}^{\mathrm{t}} \cdot\left(\frac{1}{\mathrm{t}}-\frac{1}{\mathrm{t}^2}\right) \cdot \mathrm{dt} \\ & =\frac{\mathrm{e}^{\mathrm{t}}}{\mathrm{t}}+\mathrm{c}=\frac{\mathrm{x}}{\ln \mathrm{x}}+\mathrm{c}\end{aligned}=x(\ln x)^{-1}+c$
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