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    Hi guys,

    I am struggling on this geometric series question (I have attached the image of the question and mark scheme). Why in the mark scheme have they used arn as opposed to arn-1, which I thought was the correct formula? My answer ended up being 16.24... as opposed to 15.24... which is what is in the mark scheme. Can someone please explain?

    Thanks
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    (Original post by DhylanP - CRGS)
    Hi guys,

    I am struggling on this geometric series question (I have attached the image of the question and mark scheme). Why in the mark scheme have they used arn as opposed to arn-1, which I thought was the correct formula? My answer ended up being 16.24... as opposed to 15.24... which is what is in the mark scheme. Can someone please explain?

    Thanks
    The only thing you've done wrong is to forget that, the way you've done it, n = 1 corresponds to the year 2000. Then, if we denote the year as Y, Y = n + 1999. Then, when n = 16, Y = 2015.

    The mark scheme is perhaps a little unconventional in (apparently) denoting n = 1 as the year 2001 and taking the value that year as ar^1.
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    Hi,

    I'm sorry but I don't understand exactly what you mean? Would you mind re-explaining? Sorry!

    Thanks!
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    (Original post by DhylanP - CRGS)
    Hi,

    I'm sorry but I don't understand exactly what you mean? Would you mind re-explaining? Sorry!

    Thanks!
    In your working:

    N = 1 corresponds to year 2000
    so
    N = 2 corresponds to year 2001
    N = 3 corresponds to year 2002
    etc
    N = 16 corresponds to year 2015

    From what I can see, 2015 is the required answer.
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    Hi,

    Yeah, I finally realised this. Can't believe how oblivious I was haha!
 
 
 
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