- #1
Numbnut247
- 26
- 0
Hey guys
, I need help for sum of a geometric sequence problem:
The first and second terms of a geometric sequence have a sum of 15, while the second and third terms have a sum of 60. Use an algebraic method to find the three terms.
This is what I have so far:
a + b + c
a + b = 15
b + c = 60
a = a
r = b/a
S2 = 15 = a(1-(b/a)^2)/1-(b/a)
S3 = 60 = a(1-(b/a)^3)/1-(b/a)
I then solved for a and b and got a = 3.75 and b = 11.25.
After knowing a and b, I find the common ratio: 3. But my numbers do not work for S3 because I got 48.75 (Which, interestly enough is 11.25 away from 60
)
I'm really confused
Thanks
The first and second terms of a geometric sequence have a sum of 15, while the second and third terms have a sum of 60. Use an algebraic method to find the three terms.
This is what I have so far:
a + b + c
a + b = 15
b + c = 60
a = a
r = b/a
S2 = 15 = a(1-(b/a)^2)/1-(b/a)
S3 = 60 = a(1-(b/a)^3)/1-(b/a)
I then solved for a and b and got a = 3.75 and b = 11.25.
After knowing a and b, I find the common ratio: 3. But my numbers do not work for S3 because I got 48.75 (Which, interestly enough is 11.25 away from 60
I'm really confused
![Cry :cry: :cry:](/styles/physicsforums/xenforo/smilies/cry.png)
Thanks