Answer
True.
Work Step by Step
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.
