$\int \frac{\operatorname{cosec} x d x}{\cos ^2\left(1+\log \tan \frac{x}{2}\right)}=$

$\int \frac{\operatorname{cosec} x d x}{\cos ^2\left(1+\log \tan \frac{x}{2}\right)}=$
  1. $\tan \left(1+\log \tan \frac{x}{2}\right)+\mathrm{c}$, where c is a constant of integration.
  2. $\frac{1}{2} \tan \left(1+\log \tan \frac{x}{2}\right)+\mathrm{c}$, where c is a constant of integration.
  3. $2 \tan \left(1+\log \tan \frac{x}{2}\right)+\mathrm{c}$, where c is a constant of integration.
  4. $\frac{1}{4} \tan \left(1+\log \tan \frac{x}{2}\right)+\mathrm{c}$, where c is a constant of integration.

Solution

Let $\mathrm{I}=\int \frac{\operatorname{cosec} x \mathrm{~d} x}{\cos ^2\left(1+\log \tan \frac{x}{2}\right)} \mathrm{d} x$ Let $1+\log \left(\tan \frac{x}{2}\right)=\mathrm{t}$ Differentiating both sides w.r.t. t, we get $\frac{1}{\tan \frac{x}{2}} \sec ^2 \frac{x}{2} \times \frac{1}{2} \mathrm{~d} x=\mathrm{dt}$ $\begin{aligned} & \therefore \quad \frac{1}{2 \sin \frac{x}{2} \cos \frac{x}{2}} \mathrm{~d} x=\mathrm{dt} \\ & \therefore \quad \operatorname{cosec} x \mathrm{~d} x=\mathrm{dt} \\ & \therefore \quad I=\int \frac{1}{\cos ^2 t} d t \\ & =\int \sec ^2 t d t \\ & =\tan (\mathrm{t})+\mathrm{c} \\ & =\tan \left(1+\log \left(\tan \frac{x}{2}\right)\right)+\mathrm{c} \end{aligned}$

Asked in: MHT CET 2024 (03 May Shift 2)

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