CPM Homework Help : PC3 Problem 13-65

The seventh term of an arithmetic series is $−3.2$ and the twelfth term is $2.9$ . What is the sum of the first $25$ terms of the series?

Step:

Solve the following system of equations to determine the first term and the common difference.

$−3.2 = a + d\left(7 − 1\right)$
$2.9 = a + d\left(12 − 1\right)$