Manipal Engineering Manipal Engineering Solved Paper-2008

  • question_answer
    If\[A=\left\{ x:\frac{\pi }{6}\le x\le \frac{\pi }{3} \right\}\]and \[f(x)=\cos x-x(1+x)\], then\[f(A)\]is equal to

    A) \[\left[ -\frac{\pi }{3},\,\,-\frac{\pi }{6} \right]\]

    B) \[\left[ \frac{\pi }{6},\,\,\frac{\pi }{3} \right]\]

    C) \[\left[ \frac{1}{2}-\frac{\pi }{3}\left( 1+\frac{\pi }{3} \right),\,\,\frac{\sqrt{3}}{2}-\frac{\pi }{6}\left( 1+\frac{\pi }{6} \right) \right]\]

    D) \[\left[ \frac{1}{2}+\frac{\pi }{3}\left( 1-\frac{\pi }{3} \right),\,\,\frac{\sqrt{3}}{2}+\frac{\pi }{6}\left( 1-\frac{\pi }{6} \right) \right]\]

    Correct Answer: C

    Solution :

    Since,\[f(x)\]is a continuous decreasing function on\[\left[ \frac{\pi }{6},\,\,\frac{\pi }{3} \right]\]. \[\therefore \]\[f(x)\]Attains every value between\[\left[ \frac{\pi }{6},\,\,\frac{\pi }{3} \right]\]its minimum value \[ie\],    \[f\left( \frac{1}{3} \right)=\frac{1}{2}-\frac{\pi }{3}\left( 1+\frac{\pi }{3} \right)\] and maximum value is                 \[f\left( \frac{\pi }{6} \right)=\frac{\sqrt{3}}{2}-\frac{\pi }{6}\left( 1+\frac{\pi }{6} \right)\]


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