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Math Help - arithmetic or geometric?

  1. #1
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    Question arithmetic or geometric?

    State the common difference or ratio and determine a formula for the nth term.

    5,10,20,40,...

    I got the common difference which is X 2. But I dont get the 2nd part.
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  2. #2
    MHF Contributor harish21's Avatar
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    Quote Originally Posted by blame_canada100 View Post
    State the common difference or ratio and determine a formula for the nth term.

    5,10,20,40,...

    I got the common difference which is X 2. But I dont get the 2nd part.


    The nth term of a geometric sequence with initial value a and common ratio r is given by {a_n} = r \times a^{n-1}

    So,
    The nth term for this sequence would be given by:

    {a_n} = 5 \times 2^{n-1}
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  3. #3
    A riddle wrapped in an enigma
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    Quote Originally Posted by blame_canada100 View Post
    State the common difference or ratio and determine a formula for the nth term.

    5,10,20,40,...

    I got the common difference which is X 2. But I dont get the 2nd part.
    Hi blame_canada100,

    You didn't find a common difference; you found a common ratio.

    a_n=a_1r^{n-1}

    a_n=5(2)^{n-1}

    Continue....
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