JEE Main & Advanced Mathematics Sequence & Series Question Bank Arithmetic Progression

  • question_answer
    If \[{{S}_{k}}\] denotes the sum of first \[k\]terms of an arithmetic progression whose first term and common difference are \[a\]and \[d\] respectively, then \[{{S}_{kn}}/{{S}_{n}}\] be independent of \[n\] if

    A) \[2a-d=0\]

    B) \[a-d=0\]

    C) \[a-2d=0\]

    D) None of these

    Correct Answer: A

    Solution :

    \[\frac{{{S}_{kn}}}{{{S}_{n}}}=\frac{(kn/2)\{2a+(kn-1)d\}}{(n/2)\{2a+(n-1)d\}}=k\left\{ \frac{(2a-d)+knd}{(2a-d)+nd} \right\}\] \[i.e.\] if\[2a-d=0\], then this becomes \[\frac{{{k}^{2}}nd}{nd}={{k}^{2}}\] which is obviously independent of \[n\].


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