$\left\{\begin{matrix}a=-\frac{dn}{2}+\frac{d}{2}+\frac{S}{n}\text{, }&n\neq 0\\a\in \mathrm{C}\text{, }&S=0\text{ and }n=0\end{matrix}\right.$

$S=\frac{n\left(d\left(n-1\right)+2a\right)}{2}$

$\left\{\begin{matrix}a=-\frac{dn}{2}+\frac{d}{2}+\frac{S}{n}\text{, }&n\neq 0\\a\in \mathrm{R}\text{, }&S=0\text{ and }n=0\end{matrix}\right.$