JCECE Engineering JCECE Engineering Solved Paper-2007

  • question_answer
    The maximum value of \[{{x}^{3}}-3x\] in the interval \[[0,\,\,2]\] is

    A) \[-2\]                                    

    B) \[0\]

    C) \[2\]                                     

    D) \[1\]

    Correct Answer: C

    Solution :

    Given,      \[f(x)={{x}^{3}}-3x\] \[\therefore \]  \[f'(x)=3{{x}^{2}}-3\]                                      ... (i) and        \[f'\,\,'(x)=6x\]                 ...(ii) For maxima, \[f'(x)=0\] \[\Rightarrow \]               \[3{{x}^{2}}-3=0\Rightarrow x=\pm 1\,\,\,\,\because x=1\in [0,\,\,2]\] \[\therefore \]\[x=1\]is the only solution. At           \[x=1,\,\,f'\,\,'(x)>0\] \[\therefore \]  \[x=1\]is point of minima.                 \[f(0)=,\,\,f(1)=-2\]and\[f(2)=2\] \[\therefore \]\[f(x)\]attains the maximum at \[x=2\] and maximum value\[=2\]


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