Posted by **Kelly** on Monday, January 11, 2010 at 9:04am.

Find the sum of the first four terms of the geometric sequence with a = -3 and r = 3.

I have NO idea how to attempt to solve this. I do not want an answer just instuctions. Thanks

- Algebra -
**drwls**, Monday, January 11, 2010 at 10:13am
The terms in the sequence are presumably

a*r^n.

I rceommend that you read

http://en.wikipedia.org/wiki/Geometric_progression

The first four terms would be (starting with n=0),

a, a r, ar^2 and a r^3.

The sum is a (1 + r + r^2 + r^3)

-3 * (1 + 3 + 9 + 27)

= -120

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