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Math Help - Sum question,some confusion

  1. #1
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    Sum question,some confusion

    hi everyone

    Compute S_5for the following:
    \sum^\infty _{n=0} a_n if a_n=n(n+1)(n+2)

    my working:
    start from a1= 0=0(0+1)(0+2)=0
    a2=1=1(1+1)(1+2)=6
    a3=2=2(2+1)(2+2)=24
    a4=3=3(3+1)(3+2)=60
    a5=4=4(4+1)(4+2)=120
    answer :210

    i am confused wheter the answer is 210 or 420. i started with a1(0) until a5(4).
    i am confused,please advise. did i do it right,

    thank you so much for your advice.

    thank you in advance.
    Last edited by anderson; January 29th 2010 at 08:26 PM.
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  2. #2
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    Dear anderson,

    I do not get what you mean by if

    But if your expression is ,

    a_1=1(2)(3)=6

    a_2=2(3)(4)=24

    a_3=3(4)(5)=60

    a_4=4(5)(6)=120

    a_5=5(6)(7)=210

    Therefore, S_5=6+24+60+120+210=420

    Hope this helps.
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  3. #3
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    Hi Sudharaka,

    thank you so much for helping to clarify,now i understand.

    thank you again.
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  4. #4
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    Hi Sudharaka,

    thank you so much for helping to clarify.the question is:
    \sum^\infty _{n=0} a_n if a_n=n(n+1)(n+2)
    i missed a typo.

    did i do it right???

    thank you again.
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  5. #5
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    Quote Originally Posted by anderson View Post
    Hi Sudharaka,

    thank you so much for helping to clarify.the question is:
    \sum^\infty _{n=0} a_n if a_n=n(n+1)(n+2)
    i missed a typo.

    did i do it right???

    thank you again.
    Dear anderson,

    Do you want to calculate \sum_{n=0}^{\infty}a_n or is it \sum_{n=0}^{5}a_n.

    Because, \sum_{n=0}^{\infty}a_n=\infty
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  6. #6
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    Hi Sudharaka

    the question is compute S_5, the sum of 5

    thank you & regards
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  7. #7
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    Hi Sudharaka

    the question is compute S_5, the sum of 5


    Compute S_5for the following:
    \sum^\infty _{n=0} a_n if a_n=n(n+1)(n+2)

    thank you & regards
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