JEE Main & Advanced Mathematics Applications of Derivatives Question Bank Maxima and Minima

  • question_answer
    The largest term in the sequence \[{{a}_{n}}=\frac{{{n}^{2}}}{{{n}^{3}}+200}\] is given by

    A)            \[\frac{529}{49}\]

    B)            \[\frac{8}{89}\]

    C)            \[\frac{49}{543}\]

    D)            None of these

    Correct Answer: C

    Solution :

               Consider the function            \[f(x)=\frac{{{x}^{2}}}{({{x}^{3}}+200)}\]                                   .....(i)            \[f'(x)=x\frac{(400-{{x}^{3}})}{{{({{x}^{3}}+200)}^{2}}}=0\]            When        \[x={{(400)}^{1/3}}\ ,\ (\because x\ne 0)\]                    \[x={{(400)}^{1/3}}-h\Rightarrow f'(x)>0\]                    \[x={{(400)}^{1/3}}+h\Rightarrow f'(x)<0\]            \[\therefore \]\[f(x)\] has maxima at \[x={{(400)}^{1/3}}\]            Since \[7<{{(400)}^{1/3}}<8,\]either \[{{a}_{7}}\]or \[{{a}_{8}}\]is the greatest term of the sequence.            \[\because {{a}_{7}}=\frac{49}{543}\]and\[{{a}_{8}}=\frac{8}{89}\]and \[\frac{49}{543}>\frac{8}{89}\]            \[\therefore \] \[{{a}_{7}}=\frac{49}{543}\]is the greatest term.


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