Let's first work out the pattern. Since it's an geometric progression, we can find the sum to n terms of the pattern.
[tex]S_n = \frac{a(r^n - 1)}{r - 1}[/tex]
[tex]S_n = \frac{9((-2)^n - 1)}{-3}[/tex]
[tex]S_n = -3((-2)^n - 1)[/tex]
Now that we've developed our geometric progression, we can start to add sigma notation.
k
∑(-3[(-2)^n - 1])
n = 1
