JEE Main & Advanced Mathematics Sequence & Series Question Bank Mock Test - Sequences and Series

  • question_answer
    If \[{{S}_{n}}\]denotes the sum of first n terms of an A.P. whose first term is a and \[{{S}_{nx}}/{{S}_{x}}\]is independent of x, then \[{{S}_{p}}=\]

    A) \[{{p}^{3}}\]             

    B) \[{{p}^{2}}a\]

    C) \[p{{a}^{2}}\]

    D) \[{{a}^{3}}\]

    Correct Answer: B

    Solution :

    [b] \[\frac{{{S}_{nx}}}{{{S}_{x}}}=\frac{\frac{nx}{2}[2a+(nx-1)d]}{\frac{x}{2}[2a+(x-1)d]}=\frac{n[(2a-d)+nxd]}{(2a-d)+xd}\] For \[\frac{{{S}_{nx}}}{{{S}_{x}}}\]to be independent of x, 2a-d=0 or 2a=d Now, \[{{S}_{p}}=\frac{p}{2}[2a+(p-1)d]={{p}^{2}}a\]


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