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Math Help - Change repeating decimals to fractions

  1. #1
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    Change repeating decimals to fractions

    Will you please change the following repeating decimals to fractions:
    1) 0.716 (1 is repeting) and 2) 4.23 ( 23 is repeating)

    Thank you
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  2. #2
    Senior Member DivideBy0's Avatar
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    Quote Originally Posted by oceanmd View Post
    Will you please change the following repeating decimals to fractions:
    1) 0.716 (1 is repeting) and 2) 4.23 ( 23 is repeating)

    Thank you
    In your first example I don't quite understand how the one can repeat if the 6 is after it, so I'll do the second one and hopefully you can use the method to figure out the first.

    Let x = 4.2323...

    Then 100x = 423.2323...

    So, by subtraction, we have 99x = 419 (notice how the repeating cancels itself out when we subtract, this is very important)

    So the fraction you want is x = \frac{419}{99}

    It so happens that this is the same method used to prove that 0.999... = 1. Can you prove this as well?
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  3. #3
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    Quote Originally Posted by DivideBy0 View Post
    It so happens that this is the same method used to prove that 0.999... = 1. Can you prove this as well?
    It is not really a full proof. The real proof* is using infinite series which you can find in the .999=1 thread.


    *)It is not really a proof it is a definition.
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  4. #4
    Senior Member DivideBy0's Avatar
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    Quote Originally Posted by ThePerfectHacker View Post
    It is not really a full proof. The real proof* is using infinite series which you can find in the .999=1 thread.


    *)It is not really a proof it is a definition.
    I don't see why you have to bring out the geometric series to deal with something so trivial.

    The algebraic 'proof' follows all the axioms, and so it is perfect in every respect; I don't see how you could dismiss it as a definition unless you are willing to dismiss the validity of algebra.
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  5. #5
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    Quote Originally Posted by DivideBy0 View Post
    The algebraic 'proof' follows all the axioms, and so it is perfect in every respect; I don't see how you could dismiss it as a definition unless you are willing to dismiss the validity of algebra.
    What axioms?

    Ask thyself the question: "What does .53535353.... mean?".

    You are assuming the following:
    1)If you multiply the decimal by 10^k then you move the decimal k places.
    2)If you add/subtract decimals you can do it componentwise (meaning first with first, second with second, ... )

    To prove #1 and #2 you should use geometric series.

    This is Mine 79th Post!!!
    Last edited by ThePerfectHacker; November 28th 2007 at 12:32 PM.
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  6. #6
    Senior Member DivideBy0's Avatar
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    Haha it seems my axioms were all imaginary
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  7. #7
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    Quote Originally Posted by DivideBy0 View Post
    Haha it seems my axioms were all imaginary
    It is okay to do what you did while you are in high school. But when you get older you will start to think mathematically differently. By differently I mean more formally.
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  8. #8
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    change decimals to fractions

    Thank you so very much for explaining this. I would think then the first problem should be 0.716 (716 repeating) and the fraction is 716/999

    But how to solve it using the geometric series? because this is what we study now.

    Thanks again.
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  9. #9
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    Quote Originally Posted by oceanmd View Post
    Thank you so very much for explaining this. I would think then the first problem should be 0.716 (716 repeating) and the fraction is 716/999

    But how to solve it using the geometric series? because this is what we study now.

    Thanks again.
    Hello,

    probably you have to do 0.7161616161616... = \frac{358}{495}

    to #2 and the geometric serie:

    4.23232323232323... = 4 + \frac{23}{100}+ \frac{23}{10000}+ \frac{23}{1000000}+ ...= 4 + 23\left(\frac{1}{10^2}+ \frac{1}{10^4}+ \frac{1}{10^6}+ ...\right)

    I'm sure that you now easily see the geometric serie.
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