SOLUTION: Find the sum of the first 4 terms of the geometric series where t3 = -81 and r = 3.

Algebra ->  Sequences-and-series -> SOLUTION: Find the sum of the first 4 terms of the geometric series where t3 = -81 and r = 3.       Log On


   

Question 758153: Find the sum of the first 4 terms of the geometric series where t3 = -81 and r = 3.

Answer by josgarithmetic(39618) About Me  (Show Source):
You can put this solution on YOUR website!
Sequence: a, a%2A3, a%2A3%2A3, a%2A3%2A3%2A3,...

t%5B3%5D=-81=a%2A3%2A3
%28-1%293%2A3%2A3%2A3=a%2A3%2A3
highlight%28a=-9%29

Now you can find the first four terms and find their sum.

-9, -9*3, -9*3*3, -9*3*3*3
sum: -9-9%2A3-9%2A9-9%2A27
=-9%281%2B3%2B9%2B27%29 and there is a formula if you want it, but even without,
=-9%2840%29
=-360