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Thread: arithmetic sequences formula

  1. #1
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    Question arithmetic sequences formula

    arithmetic sequences formula-ffff.png
    So I have this problem. I've got an=a1+2(n-1) but in the answer they have an=n(n-1)
    And I'm kind of confused.
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  2. #2
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    Re: arithmetic sequences formula

    according to the recursive formula

    $\displaystyle a_{n+1}-a_n=2n$

    $\displaystyle a_n-a_{n-1}=2(n-1)$

    $\displaystyle a_{n-1}-a_{n-2}=2(n-2)$

    etc.

    so $\displaystyle a_n-a_1=2(n-1)$ is not correct
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  3. #3
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    Re: arithmetic sequences formula

    Quote Originally Posted by nyanyona View Post
    Click image for larger version. 

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    So I have this problem. I've got an=a1+2(n-1) but in the answer they have an=n(n-1)
    And I'm kind of confused.
    Look HERE for the solution.
    Thanks from topsquark and nyanyona
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  4. #4
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    Re: arithmetic sequences formula



    $a_{n+1} = a_n+2n$


    $\begin{matrix}
    1 & 2 & 3 & 4 & 5& ... &n\\
    0 & 2 & 6 & 12 & 20 & ...\\
    0\cdot1&1\cdot2&2\cdot3&3\cdot4&4\cdot5&...&(n-1)n
    \end{matrix}$
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