10.3 geometric quiz
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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
