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# Need help with this GP question! watch

1. What is the 7th term of a GP if the difference between between it's 3rd term and 1st termis 18 and that between it's 4th and 2nd is 9?
2. (Original post by a_09)
What is the 7th term of a GP if the difference between between it's 3rd term and 1st termis 18 and that between it's 4th and 2nd is 9?
What have you tried?

The nth term of a GP is where a is the first term and r is the common ratio.
3. (Original post by BobbJo)
What have you tried?

The nth term of a GP is where a is the first term and r is the common ratio.
I've just tried to make these equations:-
a(r)^2 - a=18 ----(i)
ar - ar^3=9 ----(ii)
But I don't know what to do next 😓
4. (Original post by a_09)
I've just tried to make these equations:-
a(r)^2 - a=18 ----(i)
ar - ar^3=9 ----(ii)
But I don't know what to do next 😓
(i) can be rewritten as a(r^2-1)=18
(ii) can be rewritten as ar(1-r^2)=9 or -ar(r^2-1)=9

Can you see what the next step should be?
5. (Original post by BobbJo)
(i) can be rewritten as a(r^2-1)=18
(ii) can be rewritten as ar(1-r^2)=9 or -ar(r^2-1)=9

Can you see what the next step should be?
Okay, I'm trying.
6. Oh yess! I got it! Thanks a loads!! 😃

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