What is the formula for the following arithmetic sequence? - 9, - 2, 5, 12, ...

Short Answer: an = a1 + (n - 1) * d where a1 = - 9 and d = 7

Formula

an = a1 + (n - 1) * 7 We have to try it.

Givens

a4 = ?

a1 = - 9

n = 4

d = 7

Sub and solve

a4 = - 9 + (4 - 1) * 7

a4 = - 9 + 3*7

a4 = - 9 + 21

a4 = 12 which seems to check.

Answer an = a1 + (n - 1) * d