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12.3: Арифметичні послідовності
https://ukrayinska.libretexts.org/%D0%9C%D0%B0%D1%82%D0%B5%D0%BC%D0%B0%D1%82%D0%B8%D0%BA%D0%B0/%D0%90%D0%BB%D0%B3%D0%B5%D0%B1%D1%80%D0%B0/%D0%9F%D1%80%D0%BE%D0%BC%D1%96%D0%B6%D0%BD%D0%B0_%D0%B0%D0%BB%D0%B3%D0%B5%D0%B1%D1%80%D0%B0_(OpenStax)/12%3A_%D0%9F%D0%BE%D1%81%D0%BB%D1%96%D0%B4%D0%BE%D0%B2%D0%BD%D0%BE%D1%81%D1%82%D1%96%2C_%D1%80%D1%8F%D0%B4%D0%B8_%D1%82%D0%B0_%D0%B1%D1%96%D0%BD%D0%BE%D0%BC%D1%96%D0%B0%D0%BB%D1%8C%D0%BD%D1%96_%D1%82%D0%B5%D0%BE%D1%80%D0%B5%D0%BC%D0%B8/12.03%3A_%D0%90%D1%80%D0%B8%D1%84%D0%BC%D0%B5%D1%82%D0%B8%D1%87%D0%BD%D1%96_%D0%BF%D0%BE%D1%81%D0%BB%D1%96%D0%B4%D0%BE%D0%B2%D0%BD%D0%BE%D1%81%D1%82%D1%96
\(\begin{aligned} &S_{n}= a_{1} \quad+\left(a_{1}+d\right)+\left(a_{1}+2 d\right)+\ldots+a_{n} \\+&S_{n} =a_{n} \quad+\left(a_{n}-d\right)+\left(a_{n}-2 d\right)+\ldots+a_{1} \\ \hline \\ &2S_{n}=(a_{... $\begin{aligned} &S_{n}= a_{1} \quad+\left(a_{1}+d\right)+\left(a_{1}+2 d\right)+\ldots+a_{n} \\+&S_{n} =a_{n} \quad+\left(a_{n}-d\right)+\left(a_{n}-2 d\right)+\ldots+a_{1} \\ \hline \\ &2S_{n}=(a_{1}+a_{n})+(a_{1}+a_{n})+(a_{1}+a_{n})+\dots+(a_{1}+a_{n}) \end{aligned}$