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1. C2 Geometric series help
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I'm doing the January 2005 C2 paper, and I've completely forgotten how to work out the first term of the series.
The question:
6) The second and fourth terms of a geometric series are 7.2 and 5.832 respectively.
The common ratio of the series is positive. Find
a) the common ratio
b) the first term
c)...
d)...

I've worked out the common ratio ( 5.832/7.2=√0.81=0.9 ) but I need help on b). In the answers it says a=7.2/a but how does that work if I'm trying to work out a?
Thanks!
Last edited by jaz.; 21-05-2012 at 19:16.
2. Re: C2 Geometric series help
Are you sure it doesn't say a=7.2/r
3. Re: C2 Geometric series help
If you have the value of r, and you know that U2=ar then substitute in values of U2 and r to find a.
4. Re: C2 Geometric series help
(Original post by TenOfThem)
Are you sure it doesn't say a=7.2/r
It didn't, but I just checked and that gave me the answer I was looking for thank youuu
5. Re: C2 Geometric series help
(Original post by jaz.)
I'm doing the January 2005 C2 paper, and I've completely forgotten how to work out the first term of the series.
The question:
6) The second and fourth terms of a geometric series are 7.2 and 5.832 respectively.
The common ratio of the series is positive. Find
a) the common ratio
b) the first term
c)...
d)...

I've worked out the common ratio ( 5.832/7.2=√0.81=0.9 ) but I need help on b). In the answers it says a=7.2/a but how does that work if I'm trying to work out a?
Thanks!
Won't it be better that you revise your notes/book to remember the things you have forgotten rather then asking us.

Sorry if i am harsh.
6. Re: C2 Geometric series help
(Original post by fayled)
If you have the value of r, and you know that U2=ar then substitute in values of U2 and r to find a.
ahh that makes sense, I'll try this thanks
7. Re: C2 Geometric series help
(Original post by raheem94)
Won't it be better that you revise your notes/book to revise the things you have forgotten rather then asking us.

Sorry if i am harsh.
You were being honest, I would of said the same thing, but I did check my books, I checked online and I looked in a C2 text book before making an account on here just to ask that question. I guess I thought that I wouldn't need to take notes for something so simple when I was learning it.
8. Re: C2 Geometric series help
(Original post by jaz.)
You were being honest, I would of said the same thing, but I did check my books, I checked online and I looked in a C2 text book before making an account on here just to ask that question. I guess I thought that I wouldn't need to take notes for something so simple when I was learning it.
Doesn't your book has similar examples?
9. Re: C2 Geometric series help
(Original post by jaz.)
You were being honest, I would of said the same thing, but I did check my books, I checked online and I looked in a C2 text book before making an account on here just to ask that question. I guess I thought that I wouldn't need to take notes for something so simple when I was learning it.
But, the definition of a Geometric Series shows that it is a, ar, ar^2 etc

So surely you knew that 7.2=ar