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

Question

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

Answer 1

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