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Math Help - Arithmetic Series

  1. #1
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    Arithmetic Series

    tn+1 = tn + 5.5 , t1 - 2.5
    Find S5, S10, S20

    I don't know how to do this

    The previous question was -4n+15, n {1, 2, 3...} Find S5, S10, Sn
    --> I knew how to do that one but I don't really understand the equation they gave in the second one and what to do with it
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  2. #2
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    Quote Originally Posted by juliak View Post
    tn+1 = tn + 5.5 , t1 - 2.5
    Find S5, S10, S20

    I don't know how to do this

    The previous question was -4n+15, n {1, 2, 3...} Find S5, S10, Sn
    --> I knew how to do that one but I don't really understand the equation they gave in the second one and what to do with it
    The plain old boring way:

    t1 = 2.5

    t2 = t1 + 5.5 = 2.5 + 5.5 = 8

    t3 = t2 + 5.5 = 8 + 5.5 = 13.5

    etc.

    Or you could note that tn+1 - tn = 5.5 and so there's the same difference between two successive terms. So you have an arithmetic series:

    First term = 2.5

    Common difference = 5.5

    Therefore tn = 2.5 + (n-1)(5.5) = ....
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