# Math

The fourth term of a geometric series is 30; the ninth term is 960. Determine the sum of the first nine terms. How do I find t1 and r?

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1. Just translate the English using your definitions of a GS
ar^3 = 30
ar^8 = 960

divide the 2nd by third, the a's will cancel
r^5 = 32
r = 2

in ar^3 = 30
a(8) = 30
a = 15/4

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