# arithmetic series

An arithmetic series is the series, $\sum_{i=1}^{n}a_{i}$, in which each real term has the form $a_{i}=a_{i-1}+d$ for $i=2,\ldots,n$ where $d$ is constant. The sum of the sequence is given by the following $\displaystyle\frac{1}{2}n[2a_{1}+d(n-1)].$ In order to find the formula above firstly we express the terms of the sequence, $a_{2},\ldots,a_{n}$ in terms of $a_{1}$ and the constant $d$. In this case we get $a_{2}=a_{1}+d,a_{3}=a_{2}+2d,\ldots,a_{n}=a_{1}+(n-1)d$. Now we express the sum of the sequence by developing the series forward and we have:

 $S_{n}=\sum_{i=1}^{n}a_{i}=a_{1}+a_{1}+d+\cdots+a_{1}+(n-2)d+a_{1}+(n-1)d$

Reversely, we develop the series backwards and we get

 $S_{n}=a_{n}-d+a_{n}-2d+\cdots+a_{n}-(n-1)d$

It is easily seen that by adding the two expressions we get

 $\displaystyle 2S_{n}=n(a_{1}+a_{n})$ (1) $\displaystyle S_{n}=\frac{1}{2}n(a_{1}+a_{n})$ (2)

Hence, by substituting $a_{n}=a_{1}+(n-1)d$ we get the first formula.

Title arithmetic series ArithmeticSeries 2013-03-22 16:17:58 2013-03-22 16:17:58 georgiosl (7242) georgiosl (7242) 10 georgiosl (7242) Definition msc 40A05