J & K CET Engineering J and K - CET Engineering Solved Paper-2006

  • question_answer
    The set \[\{{{x}^{3}}-12x:-3\le x\le 3\}\] is equal to

    A)  \[\{x:-16\le x\le 16\}\]

    B)  \[\{x:-12\le x\le 12\}\]

    C)  \[\{x:-9\le x\le 9\}\]

    D)  \[\{x:0\le x\le 10\}\]

    Correct Answer: A

    Solution :

    Let \[y={{x}^{3}}-12x\] \[\frac{dy}{dx}=3{{x}^{2}}-12\] Put  \[\frac{dy}{dx}=0,\,\,3{{x}^{2}}-12=0\] \[\Rightarrow \] \[x=\pm 2\] At \[x=2,\,y={{2}^{3}}-12(2)=-16\] At \[x=-2,\,y={{(-2)}^{3}}-12(-2)=16\] Hence, option [a] is correct.


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