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C2 A-Level Maths - Geometric Series Watch

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    I am trying to do question 10) iv) in this paper

    http://www.ocr.org.uk/Images/63180-q...hematics-2.pdf

    It involves finding the sum of the GP for cycling
    I have r=1.1 a=2 n=30
    In the formula book I have the sum of a GP as
    Sn= a(1-r^n)/(1-r)
    So plugging in I have got = 2(1-1.1^30)/(1.1.1)

    However in the markscheme it says
    2(1.1^30-1)/(1.1-1)

    Why is this? :/
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    (Original post by Chenice)
    I am trying to do question 10) iv) in this paper

    http://www.ocr.org.uk/Images/63180-q...hematics-2.pdf

    It involves finding the sum of the GP for cycling
    I have r=1.1 a=2 n=30
    In the formula book I have the sum of a GP as
    Sn= a(1-r^n)/(1-r)
    So plugging in I have got = 2(1-1.1^30)/(1.1.1)

    However in the markscheme it says
    2(1.1^30-1)/(1.1-1)

    Why is this? :/
    2(1-1.1^30)/(1-1.1) is the same as 2(1.1^30-1)/(1.1-1)

    All that has been done is you have multiplied top and bottom of the equation by -1

    In essence the formulae can either be written:

    Sn= a(1-r^n)/(1-r)

    or

    Sn= a(r^n-1)/(r-1)
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    (Original post by Jai Sandhu)
    2(1-1.1^30)/(1-1.1) is the same as 2(1.1^30-1)/(1.1-1)

    All that has been done is you have multiplied top and bottom of the equation by -1

    In essence the formulae can either be written:

    Sn= a(1-r^n)/(1-r)

    or

    Sn= a(r^n-1)/(r-1)
    Oh ok thank you!
 
 
 
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