Solve for S_n
S_{n}=\frac{n\left(a_{1}+a_{n}\right)}{2}
Solve for a_1
\left\{\begin{matrix}a_{1}=-a_{n}+\frac{2S_{n}}{n}\text{, }&n\neq 0\\a_{1}\in \mathrm{R}\text{, }&S_{n}=0\text{ and }n=0\end{matrix}\right.
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