Question

What is the sum of an 8–term geometric series if the first term is 15 and the last term is –4,199,040?
I know how to find sum with the equation I have but I can't find common ratio. Can someone please explain how to find it.

Answer 1

the common ratio can be determined as:

an = a1 r^(n-1) , assuming we start at n=1

divide by a1, and root off the n-1 to determine r

(n-1)rt (an/a1) = r

Answer 2

n=8

a1 =15

a8 = –4,199,040
-------------------------

(8-1)rt (a8/a1) = r

7rt ( –4,199,040/15) = r

Answer 3

Thank you I get it now :)

Answer 4

:) good luck