# c2 geometric sequences questionWatch

#1
cant seem to answer this question:

in a geometric progression, the second term is -4 and the sum to infinity is 9, find the common ratio?

i can see that a= -4/r and sum to infinity = a/1-r

the answer is -1/3 (teacher gave us the answers) but i cant get to that. any help would be appreciated
0
quote
7 years ago
#2
a = -4/r is correct

You also know the sum to infinity formula is a/1-r and you're told in the question that this equals 9

So you know a/(1-r) = 9

You have two equations and two unknowns, so what do you do?
0
quote
7 years ago
#3
What answer do you get? You have enough information to do the question; substitute into and solve for (it's probably easiest to multiply through by first).
quote
7 years ago
#4
How do you know r= -1/3 as opposed to 4/3?
0
quote
7 years ago
#5
(Original post by xHuStLeR23)
How do you know r= -1/3 as opposed to 4/3?
Because we need for the sum to infinity to converge.
quote
#6
Thanks for the help guys, had a bit of a blank and couldnt see that it was a quadratic giving r=4/3 and r=-1/3. i have no idea why it cant be 4/3, maybe my teacher made a mistake
0
quote
7 years ago
#7
(Original post by matt730)
i have no idea why it cant be 4/3
Work out the 10th, 100th and 1000th terms when it's -1/3, and again when it's 4/3, and you'll see why.
quote
#8
Work out the 10th, 100th and 1000th terms when it's -1/3, and again when it's 4/3, and you'll see why.
ahhh now it makes sense. Its been a long day of c2 revision lol
0
quote
X

new posts
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• University of Warwick
Sat, 20 Oct '18
• University of Sheffield
Sat, 20 Oct '18
• Edge Hill University
Faculty of Health and Social Care Undergraduate
Sat, 20 Oct '18

### Poll

Join the discussion

#### Who is most responsible for your success at university

Mostly me (669)
90.77%
Mostly my university including my lecturers/tutors (68)
9.23%