An

**arithmetic series**is the sum of a sequence of numbers in which the difference between each number and its predecessor is a constant. The sum

*S*of the first

*n*values of a finite sequence is given by the formula:

*S*= ½*n*(*a*_{1}+*a*_{n})

*a*

_{1}is the first term and

*a*

_{n}the last.