Posted by **mike** on Sunday, May 10, 2009 at 3:33pm.

if an is a geometric sequence where a1=4 and the sum of an=12. how would you find the common ratio of the sequence? please help

- math -
**drwls**, Sunday, May 10, 2009 at 3:49pm
You must mean the infinite sum of ALL an, up to infinity. Partial sums cannot all be the same.

Let the common ratio be x.

4(1 + x + x^2 + ...) = 12

(1 + x + x^2 + ...) = 3

Now, recognize that the infinite series adds up to 1/(1-x).

1/(1-x) = 3

1/x = 1/3

x = 2/3

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