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

  • question_answer
    \[\int_{{}}^{{}}{\frac{\sin x\cos x}{a{{\cos }^{2}}x+b{{\sin }^{2}}x}dx=}\]            [AI CBSE 1988, 89]

    A)            \[\frac{1}{2(b-a)}\log (a{{\cos }^{2}}x+b{{\sin }^{2}}x)+c\]

    B)            \[\frac{1}{b-a}\log (a{{\cos }^{2}}x+b{{\sin }^{2}}x)+c\]

    C)            \[\frac{1}{2}\log (a{{\cos }^{2}}x+b{{\sin }^{2}}x)+c\]                

    D)            None of these

    Correct Answer: A

    Solution :

                       Put \[a{{\cos }^{2}}x+b{{\sin }^{2}}x=t\]\[\Rightarrow 2(b-a)\sin x\cos x=dt,\]            then \[\int_{{}}^{{}}{\frac{\sin x\cos x\,dx}{a{{\cos }^{2}}x+b{{\sin }^{2}}x}=\frac{1}{2(b-a)}\int_{{}}^{{}}{\frac{1}{t}\,dt}}\]                    \[=\frac{1}{2(b-a)}\log (a{{\cos }^{2}}x+b{{\sin }^{2}}x)+c\].


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