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Math Help - Number Sequence

  1. #1
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    Number Sequence

    Deduce the rth term of the series 5+55+555+5555+.... can be expressed as \frac{5}{9}(10^r-1).

    Hence find, in terms of k, an expression for the sum of the first k terms of the series 5+55+555+5555+....


    I tried to find the common difference or common ratio to see if it is an arithmetic or geometric progression but it didnt work out....
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  2. #2
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    The sequence 5, 55, 555, ... is neither an arithmetic nor a geometric progression. Rather, the rth element is itself (a multiple of) the sum of a geometric progression 5(10^0 + 10^1 + ... + 10^{r-1}).
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  3. #3
    MHF Contributor chisigma's Avatar
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    Quote Originally Posted by Punch View Post
    Deduce the rth term of the series 5+55+555+5555+.... can be expressed as \frac{5}{9}(10^r-1).

    Hence find, in terms of k, an expression for the sum of the first k terms of the series 5+55+555+5555+....


    I tried to find the common difference or common ratio to see if it is an arithmetic or geometric progression but it didnt work out....
    You have to remember the formula of the sum of n terms of a geometric series...

    (1)

    ... so that the r-th term of your series is obtained from (1) setting and ...

    (2)

    Now observing (2) You easily conclude that the series is the sum of two series, one 'geometric' so that You can apply again (1) obtaining...

    (3)

    Kind regards

    \chi \sigma
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