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Thread: Expression and value of Sn

  1. #1
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    Expression and value of Sn

    a) Write an expression for Sn, for the series 1 + 7 + 13 + ...

    b) Hence, find the value of n for which Sn=833

    Please help me.... I don't even know where to begin.
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  2. #2
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    Re: Expression and value of Sn

    Quote Originally Posted by teraslintu View Post
    a) Write an expression for Sn, for the series 1 + 7 + 13 + ...
    b) Hence, find the value of n for which Sn=833
    $\begin{gathered} \sum\limits_{k = 0}^n {(1 + 6k)} = 6\sum\limits_{k = 0}^n k + \sum\limits_{k = 0}^n 1 \\
    = 6\frac{{n(n + 1)}}{2} + (n + 1) \\
    = 3n(n + 1) + (n + 1) \\
    = 3{n^2} + 4n + 1 \\
    \end{gathered}$
    Thanks from topsquark
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