Write the sum using summation notation, assuming the suggested pattern continues. 1 - 3 + 9 - 27 +

Σ-3^(n-1)  for n=[1,n]

Angie above is correct though as there is no sum for this as an infinite series as r^2>1.

The only time infinite series have a sum is if r^2<1, and that sum is just s=a/(1-r)