KVPY Sample Paper KVPY Stream-SX Model Paper-17

  • question_answer
    Let \[\sum\limits_{r=1}^{n}{{{r}^{4}}=f(n)}\]. Then \[\sum\limits_{r=1}^{n}{{{(2r-1)}^{4}}}\]is equal to

    A) \[f(2n)-16f(n)\operatorname{for}\,al\operatorname{l}\,n\in N\]

    B)  \[f(n)-16f\left( \frac{n-1}{2} \right)\]when \[n\]is odd

    C) \[f(n)-16f\left( \frac{n}{2} \right)\]when \[n\]is even

    D) None of these

    Correct Answer: A

    Solution :

    \[\sum\limits_{r=1}^{n}{{{(2r-1)}^{4}}={{1}^{4}}+{{3}^{4}}+{{5}^{4}}+.....+{{(2n-1)}^{4}}}\]
    \[={{1}^{4}}+{{2}^{4}}+{{3}^{4}}+....+{{(2n)}^{4}}-[{{2}^{4}}+{{4}^{4}}+....+{{(2n)}^{4}}]\]
    \[=f(2n)-16\{{{1}^{4}}+{{2}^{4}}+....+{{n}^{4}}\}\]
    \[=f(2n)-16f(n)\]for all \[n\in N;\]
    Whether \[n\]is even on odd.


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