## Precalculus: Mathematics for Calculus, 7th Edition

See p. 855, Partial Sums of an Arithmetlc Sequence ... the nth partial sum is given by either of the following equivalent formulas: 1. $S_{n}=\displaystyle \frac{n}{2}[2a+(n-1)d]$ 2. $S_{n}=n(\displaystyle \frac{a+a_{n}}{2})$ --------------- Formula 2 : the nth partial sum is the average (arithmetic mean) of the first and list terms, times n. True.