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Math Help - 2 question i want check the answer .

  1. #1
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    2 question i want check the answer .

    Hi
    2 question i want check the answer .




    thanks
    Last edited by r-soy; April 23rd 2010 at 04:32 AM.
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  2. #2
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    Quote Originally Posted by r-soy View Post
    Hi
    2 question i want check the answer .




    thanks
    Can't you check your answers against those in the back of the book ....?
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  3. #3
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    You are correct that these are both questions about arithmetic series. But your formulas are incorrect so your answers are wrong. You need a better undersanding of what each term of the formulas actually do. Anyway, here are the correct formulas:
    a_n = a_1 + (n-1)d
    S_n = \frac{a_1 + a_n}{2}(n)
    In both problems, you know the sum S_n as well as u_1 and d. You can plug the first formula into the second to get rid of the a_n, which would make n the only variable. In solving the equation for n, you should see that it is a quadratic equation: 0 = \frac{d}{2}n^2 + (u_1 - \frac{d}{2})n - S_n. The simpliest way to solve these will be by using the quadratic formula.

    The answers I get are 20 months and 5 years.
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  4. #4
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    Quote Originally Posted by pflo View Post
    The answers I get are 20 months and 5 years.
    In other words, n = 20 (part 1) and n = 5 (part 2)
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  5. #5
    Member pflo's Avatar
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    Quote Originally Posted by Wilmer View Post
    In other words, n = 20 (part 1) and n = 5 (part 2)
    That is what I got.
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  6. #6
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    Yes, I realise that Pflo; wanted to make sure R-soy realised it too:
    he has 79.2268 and 43, quite a ways off.
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