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Arithmetic progression help watch

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    The sum of the first n terms of a series in n(n+2). Find the first three terms of the series?

    can anyone explain how to do this??


    thanks in advance
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    What are you having difficulty with here? Substitue n=1, n=2 and n=3 into n(n+2) individually. I'll do n=1 for you...
    n(n+2) = 1(1+2) = 3
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    That won't give the first, second and third elements straight off, you must do something with them.

    f(n) = n(n+2)
    f(1) = 3
    f(2) = 8
    f(3) = 15

    Now, f(n) produces the sum of the first n terms.
    So:
    Sum of the first 1 term(s) = f(1) = 3
    Sum of the first 2 terms = Second Term + f(1) = f(2) = 8
    Sum of the first 3 terms = Third term + f(2) = f(3) = 15

    Therefore:
    First term = 3
    Second Term = 8 - 3 = 5
    Third term = 15 - 8 = 7

    We can check this is right by suming the terms:
    Sum of first 1 terms = 3 = 3
    Sum of first 2 terms = 3 + 5 = 8 = 8
    Sum of first 3 terms = 3 + 5 + 7 = 15 = 15

    So we're right, it's:
    3, 5, 7.

    Therefore
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    thank you for your help we got two questions like that for homework

    stuck on the other one now but ill give it a go
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    Therefore
    ... what ?

    // -ve.
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    if my teacher tought it like that i would have had no problem lol

    thanks for making it simple
 
 
 
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Updated: November 13, 2005

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