Posted by **anonymous** on Monday, April 18, 2011 at 10:05am.

find Sn for geometric sequence with a1 = 8, n = 9, and r = -1?

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

= 8(1-1)/(1+1) = 8(0)/2

= 0

is this correct..?

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