JEE Main & Advanced Mathematics Indefinite Integrals Question Bank Integration by Parts

  • question_answer
    \[{{I}_{1}}=\int{{{\sin }^{-1}}x\,\,dx}\] and \[{{I}_{2}}=\int{{{\sin }^{-1}}\sqrt{1-{{x}^{2}}}}dx\]then                 [Kerala (Engg.) 2005]

    A)                 \[{{I}_{1}}={{I}_{2}}\]

    B)                 \[{{I}_{2}}=\pi /2{{I}_{1}}\]

    C)                 \[{{I}_{1}}+{{I}_{2}}=\pi /2x\]

    D)                 \[{{I}_{1}}+{{I}_{2}}=\pi /2\]

    E)                 \[{{I}_{1}}-{{I}_{2}}=\pi /2x\]

    Correct Answer: C

    Solution :

                    \[{{I}_{1}}=\int{{{\sin }^{-1}}xdx}\]                                 Let \[{{\sin }^{-1}}x=\theta \]Þ \[x=\sin \theta \] Þ \[dx=\cos \theta \,d\theta \]                                 \[{{I}_{1}}=\int{\theta \cos \theta d\theta }\]\[=\theta \sin \theta -\int{\sin \theta d\theta }\]\[=\theta \sin \theta +\cos \theta \]                                     \[=x{{\sin }^{-1}}x+\sqrt{1-{{x}^{2}}}\]                                 \[{{I}_{2}}=\int{{{\sin }^{-1}}\sqrt{1-{{x}^{2}}}}dx\]\[=\int{{{\cos }^{-1}}xdx}\]                                 Let \[\cos \varphi =x,\] Hence \[-\sin \varphi \,d\varphi =dx\]                                 \[{{I}_{2}}=-\int{\varphi \sin \varphi d\varphi }\]\[=\varphi \cos \varphi +\int{-\cos \varphi d\varphi }\]                                      \[=\varphi \cos \varphi -\sin \varphi \]\[=x{{\cos }^{-1}}x-\sqrt{1-{{x}^{2}}}\]                                 \[{{I}_{1}}+{{I}_{2}}=x({{\cos }^{-1}}x+{{\sin }^{-1}}x)=\frac{\pi }{2}x\].


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