### Home > A2C > Chapter 13 > Lesson 13.2.3 > Problem13-135

13-135.

Each of the following sums is the beginning of an arithmetic series. Find the nth term of each series, for the specified value of n.

1. $\left(−2\right) + \left(−5\right) + \left(−8\right) + …, n = 12$

Set up an equation to describe the sequence.

$t\left(n\right) = −2 − 3\left(n − 1\right)$

Now substitute 12 for n to find the 12th term.

$t\left(12\right) = −2 − 3\left(11\right)$

$35$

1. $9 + 15 + 21 + …, n = 20$

See part (a).