You are Here: Home >< Maths

# Geometric Progressions - Help please... Watch

1. What is the sum of n terms of the following series:

a) x + 1 + 1/x + ...

b) 1 - y + y^2 - ...
2. (Original post by Abbie_D)
What is the sum of n terms of the following series:

a) x + 1 + 1/x + ...

b) 1 - y + y^2 - ...
find the common ratio and use the summation formula
3. (Original post by TeeEm)
find the common ratio and use the summation formula
Thanks for the reply. I have done but I still cannot get the exact answers.
4. (Original post by Abbie_D)
Thanks for the reply. I have done but I still cannot get the exact answers.
post your working and I will try to point you in the right direction
5. (Original post by TeeEm)
post your working and I will try to point you in the right direction

My working so far:

Sn = x[(1/x)^n-1]/1/x-1
= x^n-1/1/x-1

6. (Original post by Abbie_D)

My working so far:

Sn = x[(1/x)^n-1]/1/x-1
= x^n-1/1/x-1

first line is fine

What is the answer meant to be, because the expression will be ugly no matter what
7. Answer is: xn - 1 / xn-2(x-1)
8. (Original post by Abbie_D)
Answer is: xn - 1 / xn-2(x-1)
9. (Original post by TeeEm)
No, sorry
10. (Original post by Abbie_D)
No, sorry
expand the bracket at the top
then multiply top and bottom by xn
11. (Original post by TeeEm)
expand the bracket at the top
then multiply top and bottom by xn+1
I'm still puzzled. Would it be possible if you could show your working to me please?
12. (Original post by Abbie_D)
I'm still puzzled. Would it be possible if you could show your working to me please?
[x(1/xn - 1)]/[1/x-1]

=[1/xn-1 - x]/[1/x-1]

multiply to and bottom by xn-1
13. (Original post by TeeEm)
[x(1/xn - 1)]/[1/x-1]

=[1/xn-1 - x]/[1/x-1]

multiply to and bottom by xn-1
I can understand up to: 1 - (x1 x xn-1 ) / xn-1 (1/x - 1)n
And I know how to get xn - 1 on the top, but how do you get the xn-2
(x-1) on the bottom?

I really appreciate your help by the way!
14. (Original post by Abbie_D)
I can understand up to: 1 - (x1 x xn-1 ) / xn-1 (1/x - 1)n
And I know how to get xn - 1 on the top, but how do you get the xn-2
(x-1) on the bottom?

I really appreciate your help by the way!
the bottom should look

1/xn-1[1/x-1]

=1/xn-1 1/x[1 - x]

=1/xn-2[1 - x]
15. (Original post by TeeEm)
the bottom should look

1/xn-1[1/x-1]

=1/xn-1 1/x[1 - x]

=1/xn-2[1 - x]
Thank you so much
16. (Original post by Abbie_D)
Thank you so much
my pleasure

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 29, 2015
Today on TSR

Can it be done?

### Luxury Dublin hotel bans ALL influencers

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

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

• Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

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

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE