KVPY Sample Paper KVPY Stream-SX Model Paper-26

  • question_answer
    Let f be a non-negative function defined on the interval \[\left[ 0,1 \right]\]. If \[\int_{0}^{x}{\sqrt{1-{{\left( f'\left( t \right) \right)}^{2}}}dt=\int_{0}^{x}{f(t)dt}}\] \[0\le x\le 1\] and \[f\left( 0 \right)=0\], then

    A) \[f\left( \frac{1}{2} \right)<\frac{1}{2}and\,\,f\left( \frac{1}{3} \right)>\frac{1}{3}\]

    B) \[f\left( \frac{1}{2} \right)>\frac{1}{2}and\,\,f\left( \frac{1}{3} \right)>\frac{1}{3}\]

    C) \[f\left( \frac{1}{2} \right)<\frac{1}{2}and\,f\left( \frac{1}{3} \right)<\frac{1}{3}\]

    D) \[f\left( \frac{1}{2} \right)>\frac{1}{2}and\,f\left( \frac{1}{3} \right)<\frac{1}{3}\]

    Correct Answer: C

    Solution :

    We have,\[\int_{0}^{x}{\sqrt{1-{{(f'\left( t \right))}^{2}}}dt=\int_{0}^{x}{f\left( t \right)dt}}\] On differentiating, we get
    \[\sqrt{1-{{(f'(x))}^{2}}}=f(x)\] \[\Rightarrow \,\,\,\,f'(x)=\pm \sqrt{1-{{(f(x))}^{2}}}\]
    \[\therefore \]      \[f\left( x \right)=\sin x\]
    \[f\left( x \right)\ne -\sin x\]
    \[x>\sin x\]
    \[\therefore \,\,\,\frac{1}{2}>\sin \frac{1}{2}\] \[\Rightarrow \,\,\,\,\,\frac{1}{2}>f\left( \frac{1}{2} \right)and\frac{1}{3}>f\left( \frac{1}{3} \right)\]


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