SOLUTION: find the sum of the G.P 1 + 3 + 9 + 27 + ------- + 729?

Algebra ->  Sequences-and-series -> SOLUTION: find the sum of the G.P 1 + 3 + 9 + 27 + ------- + 729?      Log On


   

Question 1046326: find the sum of the G.P 1 + 3 + 9 + 27 + ------- + 729?
Found 2 solutions by Boreal, ikleyn:
Answer by Boreal(15235) About Me  (Show Source):
You can put this solution on YOUR website!
sum is the sum of a1(1-r^(n+1))/(1-r)
3 is the common ratio, the first term is 1
The last term is 3^6, the first term is 3^0, so we use 3^(n+1) in the formula
1(1-3^7)/(-2)
-2186*(-1/2)=1093
check:
1+3+9+27+81+243+729=1093

Answer by ikleyn(52790) About Me  (Show Source):
You can put this solution on YOUR website!
.
If you don't know how to calculate the sum of a GP, look into the lessons
    - Geometric progressions
    - The proofs of the formulas for geometric progressions
    - Problems on geometric progressions
    - Word problems on geometric progressions
in this site.