Explicit Models
Example 2

Find the 15th term of an arithmetic sequence whose first term is -1 and whose common difference is 4.

Step 1. Use the formula to determine the explicit model.

an = a1+ d(n - 1)

an = -1 + 4(n - 1)

Step 2. Substitute the position of the term into the model and solve; n = 15.

a15 = -1 + 4(15 - 1)

a15 = -1 + 4(14)

a15 = 55