BCECE Engineering BCECE Engineering Solved Paper-2005

  • question_answer
    If the ratio of the sum of m and \[n\]terms of an AP is \[{{m}^{2}}:{{n}^{2}},\]then the ratio of its \[mth\]and \[nth\]terms is:

    A) \[m-1:n-1\]           

    B)         \[2m+1:2n+1\]

    C)         \[2m-1:2n-1\]

    D)         none of these

    Correct Answer: C

    Solution :

    Let a and d be the first term and common difference respectively. Given that              \[\frac{{{S}_{m}}}{{{S}_{n}}}=\frac{{{m}^{2}}}{{{n}^{2}}}\]                 \[\Rightarrow \]    \[\frac{m/2[2a+(m-1)d]}{n/2[2a+(n-1)d]}=\frac{{{m}^{2}}}{{{n}^{2}}}\] \[\Rightarrow \]    \[\frac{2a+(m-1)d}{2a+(n-1)d}=\frac{m}{n}\] Replace \[m\]by \[2m-1\]and\[n\]by \[2n-1\] \[\Rightarrow \]    \[\frac{2a+(2m-2)d}{2a+(2n-2)d}=\frac{2m-1}{2n-1}\] \[\frac{a+(m-1)d}{a+(n-1)d}=\frac{2m-1}{2n-1}\]                 \[\Rightarrow \]Ration of \[mth\]and \[nth\]terms                                 \[=(2m-1):(2n-1)\]


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