10.3 geometric quiz

Find the sum of the first 18 terms of the geometric sequence: 2, -8, 32, -128, Á .

Find common ratio first! We can find the common ratio by dividing the second term of 2, -8, 32, -128, Á by the first term. So -4

Sum of the first n terms of a geometric sequence

Sn=a1(1-r^n)/1-r

General term of geometric sequence

an=a1(r)^n-1