Question

Derive the explicit rule for the pattern: 3, 0, - 3, - 6, - 9, ...

Answer 1

Explanation:

a1 = 3

d = -3

an = a1 + (n - 1)*d

an = a1 + (n - 1)*-3

Try it

Let n = 5

a5 = 3 + (5 - 1)*-3

a5 = 3 + 4*-3

a5 = 3 - 12

a5 = - 9 which is exactly what it should be.

The interesting one to try is n = 2

a2 = a1 + (2 - 1)*-3

a3 = 3 + 1(-3)

a3 = 3 - 3

a3 =0

Which is exactly what the second term is. It's interesting because you would never guess that 0 is what you get.