JEE Main & Advanced Mathematics Differentiation Question Bank Derivative at a point Standard Differentiation

  • question_answer
    The first derivative of the function \[(\sin 2x\cos 2x\cos 3x+{{\log }_{2}}{{2}^{x+3}})\] with respect to x at \[x=\pi \]is                              [MP PET 1998]

    A)            2

    B)            ?1

    C)            \[-2+{{2}^{\pi }}{{\log }_{e}}2\]

    D)            \[-2+{{\log }_{e}}2\]

    Correct Answer: B

    Solution :

               \[f(x)=\sin 2x.\cos 2x.\cos 3x+{{\log }_{2}}{{2}^{x+3}}\]                    \[f(x)=\frac{1}{2}\sin 4x\cos 3x+(x+3){{\log }_{2}}2\]                    \[f(x)=\frac{1}{4}[\sin 7x+\sin x]+x+3\]                    Differentiate w.r.t. x,  \[f'(x)=\frac{1}{4}[7\cos 7x+\cos x]+1\]                    \[f'(x)=\frac{7}{4}\cos 7x+\frac{1}{4}\cos x+1\].                    Hence \[f'(\pi )=-2+1=-1\].


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