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