The Student Room Group
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>
Competition - post a photo you've taken of nature >>

Core 2 - Geometric Progression

Was wondering if anyone could help me out. I have the answers but one step i just can't get my head around.

In a geometric progression, the second term is -4 and the sum to infinity is 9. Find the common ratio.

I have:

ar = -4

a/1-r = 9

And then in the answers they jump from that to: -4/r = 9-9r
I just can't see why, and i'm probably being silly! Anyone help me out? :confused:
Reply 1
Avatar for 0-)
0-)
11
From the first equation: a = -4/r
From the second equation: a = 9(1-r) = 9-9r

So we have a= -4/r = 9-9r.
Reply 2
Avatar for EoghanM
EoghanM
set both equations equal to 'a' then set them equal to each other then solve!!!

your welcome
Reply 3
Avatar for livingthedream
livingthedream
OP
Thank you so much guys!

I see it now. *facepalm* I was being very thick.

:biggrin: Appreciate the help.
So what's the answer please? I ended up with two answers which can't be right (i guess I'm being thick seeing as this is last year's stuff)
Reply 5
Avatar for vinvinvin
vinvinvin
4
you know the sum to infinity is 9 so if you sub in both of your answers only one will equal 9.. this is the correct answer

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you