TeX source:

\begin{gathered} \frac{S_{n}=a_{1}+\left(a_{1}+d\right)+\left(a_{1}+2 d\right)+\ldots+\left(a_{n}-d\right)+a_{n} \\ +\quad S_{n}=a_{n}+\left(a_{n}-d\right)+\left(a_{n}-2 d\right)+\ldots+\left(a_{1}+d\right)+a_{1}} {2 S_{n}=\left(a_{1}+a_{n}\right)+\left(a_{1}+a_{n}\right)+\ldots+\left(a_{1}+a_{n}\right)} \end{gathered}