CPM Homework Help : CCA Problem 10-56

How many terms are in the arithmetic sequence shown below?

$15,\ 7,\ −1,\ −9,...,\ −225$

Step 1:

Write an equation for the arithmetic sequence using $a_n−m\left(n-1\right)+a_1$

Step 2:

Let $a_n=−225$ and solve for $n$ .