JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Increasing and Decreasing Function

  • question_answer
    If \[f(x)={{x}^{3}}-6{{x}^{2}}+9x+3\] be a decreasing function, then x lies in [RPET 2002]

    A)            \[(-\infty ,-1)\cap (3,\,\infty )\]

    B)            \[(1,\,\,3)\]

    C)            \[(3,\,\,\infty )\]

    D)            None of these

    Correct Answer: B

    Solution :

               \[f(x)={{x}^{3}}-6{{x}^{2}}+9x+3\], For decreasing \[{f}'(x)<0\]             Þ  \[3{{x}^{2}}-12x+9<0\]   Þ \[{{x}^{2}}-4x+3<0\]                    Þ  \[(x-3)\,\,(x-1)<0\],  \ \[x\in (1,\,3)\].


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