CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2001

  • question_answer
    The abscissae of the points, where the tangent to curve \[y={{x}^{3}}-3{{x}^{2}}-9x+5\]is parallel to \[x-\]axis, are:

    A)  \[x=0\]and 0   

    B)  \[x=1\]and \[-1\]

    C) \[~x=-1\] and 3                

    D)  none of these

    Correct Answer: C

    Solution :

    Equation of curve \[y={{x}^{3}}-3{{x}^{2}}-9x+5\] \[\frac{dy}{dx}=3{{x}^{2}}-6x-9\] We know that this equation gives the slope of the tangent to the curve, we also know that if the tangent is to the parallel to \[x-\]axis then\[\frac{dy}{dx}=0\] \[\Rightarrow \]\[3{{x}^{2}}-6x-9=0\] \[{{x}^{2}}-2x-3=0\] \[\Rightarrow \]               \[x=-1,3\]


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