Question

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

Answer 1

an = -9 + 7 ( n - 1)

Explanation:

Answer 2

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