JEE Main & Advanced Mathematics Indefinite Integrals Question Bank Fundamental Integration

  • question_answer
    If \[\int_{{}}^{{}}{(\sin 2x-\cos 2x)}\ dx=\frac{1}{\sqrt{2}}\sin (2x-a)+b\], then [Roorkee 1978; MP PET 2001]

    A)            \[a=\frac{\pi }{4},\ b=0\]

    B)            \[a=-\frac{\pi }{4},\ b=0\]

    C)            \[a=\frac{5\pi }{4},\ b=\]any constant                                        

    D)            \[a=-\frac{5\pi }{4},\ b=\]any constant

    Correct Answer: D

    Solution :

               \[\int_{{}}^{{}}{(\sin 2x-\cos 2x)\,dx=\frac{1}{\sqrt{2}}\sin (2x-a)+b}\]            \[\Rightarrow -\frac{1}{2}(\sin 2x+\cos 2x)=\frac{1}{\sqrt{2}}\sin (2x-a)+b\]            \[\Rightarrow -\left[ \frac{1}{\sqrt{2}}\sin 2x+\frac{1}{\sqrt{2}}\cos 2x \right]=\sin (2x-a)+b\sqrt{2}\]            \[\Rightarrow \sin \left( 2x+\frac{5\pi }{4} \right)=\sin (2x-a)+b\sqrt{2}\]            \[\Rightarrow b\] is any constant and \[a=\frac{-5\pi }{4}\].


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