## Quick Reference

A series or progression of numbers in which there is a common difference between terms, e.g. 3, 9, 15, 21,… is an arithmetic series with a common difference of 6. The general formula for the *n*th term is

[*a* + (*n* − 1)*d*]

and the sum of *n* terms is

*n*[2*a* + (*n* − 1)*d*]/2.

Compare geometric series.

*Subjects:*
Mathematics.