等差数列

$3$\displaystyle{\hspace{1}{a}_{n}=\hspace{1}{a}_{1}+ $ n- 1 $ d}$
$3$\displaystyle{\hspace{1}{a}_{1}}$ ：初項， $3$d$ ：公差

第 $3$n$ 項までの和は，
$3$\displaystyle{\begin{array}{lll} \hspace{1}{S}_{n} & =\frac{\hspace{2}1\hspace{2}}{\hspace{2}2\hspace{2}} $ \hspace{1}{a}_{1}+\hspace{1}{a}_{n} $ n & \vspace{6}\\ & =\frac{\hspace{2}n\hspace{2}}{\hspace{2}2\hspace{2}} \{ 2\hspace{1}{a}_{1}+ $ n- 1 $ d \} & \vspace{6}\\ \end{array}}$

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