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Math Help - AS Mathematics Pure Core 2 - Sequences and Series Help!

  1. #1
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    AS Mathematics Pure Core 2 - Sequences and Series Help!

    I need helping forming the equation given in the question, any help?

    The sum of the first three terms of a geometric progression is 14. If the first term is 2, and the common ratio is r , show that r2 + r - 6 = 0


    I tried substituting a = 2 and n = 3 into the geometric formula for the sum of a series, such that 2(r3 - 1) / (r - 1) = 14, but I can't get it to equal anything near the equation given in the question, so I think I'm doing something wrong...
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    Re: AS Mathematics Pure Core 2 - Sequences and Series Help!

    Quote Originally Posted by LarsaSolidor View Post
    I need helping forming the equation given in the question, any help?

    The sum of the first three terms of a geometric progression is 14. If the first term is 2, and the common ratio is r , show that r2 + r - 6 = 0


    I tried substituting a = 2 and n = 3 into the geometric formula for the sum of a series, such that 2(r3 - 1) / (r - 1) = 14, but I can't get it to equal anything near the equation given in the question, so I think I'm doing something wrong...
    2+2r+2r^2=14 solve for r~.
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    Re: AS Mathematics Pure Core 2 - Sequences and Series Help!

    Thanks for your reply; I managed to work it out about 3 minutes after posting this!
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