C2 Geometric Series
Maths and statistics discussion, revision, exam and homework help.
-
- Reputation:
- Thread Starter
- Adored and Respected Member
- Posts: 443
C2 Geometric SeriesThe sum of the geometric series 1+r+r2+... is k times the sum of the series 1-r+r2..., where k>0. Express r in terms of k.
By equating the sums of both series, I've got (1-rn)/(1-r) = k(1-(-r)n)/(1+r), but I'm struggling from here. Help appreciated. -
- Reputation:
- Thread Starter
- Adored and Respected Member
- Posts: 443
Re: C2 Geometric SeriesThe question doesn't say anything about the value of r - would you think I could assume that -1<r<1?(Original post by BabyMaths)
I think you should be looking at the sum to infinity.
Thanks -
- Reputation:
- Banned
- Location: Langley Posts: ±5
Re: C2 Geometric SeriesMight mean you multiply by(Original post by TrueGrit)
If |r| > 1 then the sum of the series is infinity, so presumable it means not.
Also are you sure about the second series: 1-r+r2 the 'minus' ?
-
- Reputation:
- Exalted and Worshipped Member
- Posts: 922
Re: C2 Geometric SeriesMore like the question most likely means a geometric series with a ratio of -r. If the minus were a plus, there wouldn't be much of a question.(Original post by TrueGrit)
I don't know I didn't write the question... -
- Reputation:
- Banned
- Location: Langley Posts: ±5
Re: C2 Geometric SeriesI meant that the common ratio is(Original post by TrueGrit)
I don't know I didn't write the question...
I think the OP's missing a part of the question out, sum to infinity maybe. -
- Reputation:
- Thread Starter
- Adored and Respected Member
- Posts: 443
Re: C2 Geometric SeriesJust re-checked the question, it's actually written "the sum of the infinite geometric series 1+r+..." but I'm not sure what it means by infinte series.(Original post by Imposition)
I meant that the common ratio is
I think the OP's missing a part of the question out, sum to infinity maybe. -
- Reputation:
- Peer Of The TSR Realm
- Posts: 1,575
Re: C2 Geometric SeriesSo, it took you 13 hours to get around to reading the question properly.
If |r|<1 then as you add terms the sum approaches some particular value. For example S = 1+1/2+1/4+1/8+1/16......
The sums are
1
1+1/2=3/2
1+1/2+1/4=7/4
1+1/2+1/4+1/8=15/8
and it's clearly approaching 2 and you can get as close to 2 as you like.Last edited by BabyMaths; 23-06-2012 at 08:43. -
- Reputation:
- Peer Of The TSR Realm
- Location: Hungary
- Posts: 1,537
Re: C2 Geometric SeriesFor infinite geometric series(Original post by Julii92)
The sum of the geometric series 1+r+r2+... is k times the sum of the series 1-r+r2..., where k>0. Express r in terms of k.
By equating the sums of both series, I've got (1-rn)/(1-r) = k(1-(-r)n)/(1+r), but I'm struggling from here. Help appreciated.
for |r|<1
So your equation
Solve for r
then from |r|<1 determine which integer can be k (maybe it's only my question)Last edited by ztibor; 24-06-2012 at 09:36.
