MCQ
$\int_0^{\pi /2} {{{\left( {\frac{\theta }{{\sin \theta }}} \right)}^2}d\theta = } $
  • $\pi \log 2$
  • B
    $\frac{\pi }{{\log 2}}$
  • C
    $\pi $
  • D
    None of these

Answer

Correct option: A.
$\pi \log 2$
a
(a) Let $I = \int_0^{\pi /2} {{{\left( {\frac{\theta }{{\sin \theta }}} \right)}^2}d\theta } $

$= [ - {\theta ^2}\cot \theta ]_0^{\pi /2} + \int_0^{\pi /2} {\,\,\,\,2\theta .\cot \theta .\,d\theta } $

$ = 2[\theta .\log \sin \theta ]_0^{\pi /2} - 2\int_0^{\pi /2} {\log \sin \theta \,d\theta } $

$ \Rightarrow \frac{I}{2} = 0 - \mathop {\lim }\limits_{\theta \to 0} \theta \log .\sin \theta $

$ - \int_0^{\pi /2} {\log \sin \theta \,d\theta } $

==> $\frac{\pi }{2}\log 2$.

Hence $I =\pi \log 2$.

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