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MHT CET · Maths · Indefinite Integration

\(\int_{\frac{\pi}{6}}^{\frac{\pi}{2}} \frac{\operatorname{cosec} x \cdot \cot x}{1+\operatorname{cosec}^2 x} d x=\)

  1. A \(\frac{\pi}{4}-\tan ^{-1} 2\)
  2. B \(\tan ^{-1} 1\)
  3. C \(\tan ^{-1} 2\)
  4. D \(\tan ^{-1}\left(\frac{1}{3}\right)\)
Verified Solution

Answer & Solution

Correct Answer

(D) \(\tan ^{-1}\left(\frac{1}{3}\right)\)

Step-by-step Solution

Detailed explanation

Let \(I=\int_{\frac{\pi}{6}}^{\frac{\pi}{2}} \frac{\operatorname{cosec} x \cdot \cot x}{1+\operatorname{cosec}^2 x} d x\)
Put \(\operatorname{cosec} x=t \Rightarrow \operatorname{cosec} x \cot x=-d t\). When \(x=\frac{\pi}{6}, t=2\) and when \(x=\frac{\pi}{2}, t=1\)
\(\mathrm{I}=\int_2^1(-1) \frac{\mathrm{dt}}{1+\mathrm{t}^2} \)
\( =\int_1^2 \frac{\mathrm{dt}}{1+\mathrm{t}^2}=[\tan ^{-1} \mathrm{t}_1^2=\tan ^{-1}(2)-\tan ^{-1}\) \((1)=\tan ^{-1}\left[\frac{2-1}{1+(2)(1)}\right]=\tan ^{-1}\left(\frac{1}{3}\right)\)