x Turn on thread page Beta
 You are Here: Home >< Maths

# Questions on infinite geometric progressions watch

1. 1) A GP has a 1st term of 6 and a sum to infinity of 60. Find the common ratio.

I've done

The answer given in the book is 9/10.

Am I using the right method? And if so, where have I gone wrong.

2) Work out each of these sums
^r+1

I've got as far as
a=(0.3)^2=0.9
Unsure what r would equal though.

Thanks.
2. (Original post by N!*)
1) A GP has a 1st term of 6 and a sum to infinity of 60. Find the common ratio.

I've done

The answer given in the book is 9/10.

Am I using the right method? And if so, where have I gone wrong.

2) Work out each of these sums
^r+1

I've got as far as
a=(0.3)^2=0.9
Unsure what r would equal though.

Thanks.

from here, multiply the denominator of 1-r to both sides.
so, 60(1-r)=6, then expand and simplify.
3. (Original post by KeineHeldenMehr)

from here, multiply the denominator of 1-r to both sides.
so, 60(1-r)=6, then expand and simplify.
Thanks , thought it might be something like that.
4. (Original post by N!*)
Thanks , thought it might be something like that.
for the 2nd question
calculate a again, i get 0.09
the geometric progression works like this: a, ar, ar^2, ar^3

so a=0.09, ar = 0.027 (by subbing into the summation formula given in the question.)

divide ar by a to get the ratio.

Spoiler:
Show
you should get r=0.3, then use the sum to infinity formula then hey presto!
5. (Original post by KeineHeldenMehr)
for the 2nd question
calculate a again, i get 0.09
the geometric progression works like this: a, ar, ar^2, ar^3

so a=0.09, ar = 0.027 (by subbing into the summation formula given in the question.)

divide ar by a to get the ratio.

Spoiler:
Show
you should get r=0.3, then use the sum to infinity formula then hey presto!
Such a great help, thankyou so much
+rep

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: February 13, 2010
Today on TSR

### Complete university guide 2019 rankings

Find out the top ten here

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