JEE Main & Advanced Sample Paper JEE Main Sample Paper-21

  • question_answer
    The length of perpendiculars from the foci S and S' on any tangent to ellipse \[\frac{{{x}^{2}}}{4}+\frac{{{y}^{2}}}{9}=1\] are a and c respectively, then the value of \[\int\limits_{-ac}^{ac}{\{2x\}\,dx}\] equal to Note; {k} denotes fractional part of k.

    A)  \[\frac{1}{4}\]                         

    B)  \[\frac{1}{2}\]

    C)  2                                

    D)  4

    Correct Answer: D

    Solution :

    We know that ac = (semi-minor axis)2 = 4 Now     \[\int\limits_{-4}^{4}{\{2x\}\,dx=}16\,\int\limits_{0}^{1/2}{\{2x\}\,dx\,=\int\limits_{-8\times \frac{1}{2}}^{8\times \frac{1}{2}}{\{2x\}\,dx}}\] \[=16\int\limits_{0}^{1/2}{\{2x\}dx\,=16}\,\int\limits_{0}^{1/2}{2x\,dx=32}\,\int\limits_{0}^{1/2}{x\,dx}\] \[=32\left( \frac{{{x}^{2}}}{2} \right)_{0}^{\frac{1}{2}}\,=32\times \frac{1}{8}=4\]


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