JEE Main & Advanced Mathematics Sequence & Series Question Bank Self Evaluation 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 \[\frac{{{S}_{nx}}}{{{S}_{x}}}\] 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] \[\frac{{{S}_{nx}}}{{{S}_{x}}}=\frac{\frac{nx}{2}[2a+\left( nx-1 \right)d]}{\frac{x}{2}[2a+\left( x-1 \right)d]}\] \[=\frac{n[(2a-d)+nxd]}{\left( 2a-d \right)+xd}\] For \[\frac{{{S}_{nx}}}{{{S}_{x}}}\]to be independent of x  \[2a-d=0\] \[\therefore \,\,\,2a=d\] now,  \[{{S}_{p}}=\frac{P}{2}[2a+(P-1)d]={{P}^{2}}a\]


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