What is the sum of the first 21 terms of this arithmetic series?
-5+(-3)+(-1)+1+...
The first term is -5
The common difference is 2
So...the 21st term = -5 + 2 (21 - 1) = -5 + 2(20) = 35
The sum, S, of the series =
( first term + last term ) ( number of terms) / 2 =
(-5 + 35) (21) /2 =
(30) * 21 / 2 =
(30/2) * 21 =
15 * 21 =
315
