SOLUTION: The sum of a finite geometric series. Given n=6 -5, -15, -45, ..., I worked this out and ended up with -1820 , but I am not sure the result is correct. a1(1−r^n)

Algebra ->  Sequences-and-series -> SOLUTION: The sum of a finite geometric series. Given n=6 -5, -15, -45, ..., I worked this out and ended up with -1820 , but I am not sure the result is correct. a1(1−r^n)      Log On


   

Question 1109416: The sum of a finite geometric series.
Given n=6
-5, -15, -45, ...,
I worked this out and ended up with -1820 , but I am not sure the result is correct.
a1(1−r^n)/1−r
-5(1-3^6/1-2 = 3640/-2 = -1820
Please let me know if the formula I used was incorrect, thanks!
Answer by Boreal(15235) About Me  (Show Source):
You can put this solution on YOUR website!
Can check the result
-5-15-45-135-405-1215=-1820
you are correct, except the denominator is 1-3 to get -2, not 1-2.