Core 2 - Geometric Progression

    • Thread Starter

    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:

    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.

    set both equations equal to 'a' then set them equal to each other then solve!!!

    your welcome
    • Thread Starter

    Thank you so much guys!

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

    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)

    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
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