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    How to get the formula for Sn of an arithmetic series?

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    Tn = a+(n -1)d

    sn = n/2(a + n)

    4. For the series 3 + 9 + 15 + 21... Find:
    a) an expression for tn
    b) an expression for sn
    c) the answer if s = 20
    d) the answer if t = 115

    5. Find the # of terms in the sequencece 2,6,10... 626

    I need a step by step procedure for solving these problems.

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    https://brainmass.com/math/algebra/get-formula-arithmetic-series-5082

    Solution Preview

    4. In this series a=3, d=6. So
    <br> a) tn=3+(n-1).6=6n-3
    <br> b) sn=n(3+n)/2
    <br> c) if s=20, then n(n+3)/2=20, ...

    Solution Summary

    Solution contains a detailed explanation on how to find the general formula for Sn of an arithmetic series.

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