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Math Help - Why this last step when changing the subject?

  1. #1
    Junior Member
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    Derbyshire, central UK
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    Why this last step when changing the subject?

    Hi, I'm really pleased to say that I have learned lots from your answers, thank you again. I do not understand why the last step is taken in the question below. (Change the subject of the formula to y)

    x-2y=10

    x-10=2y

    2y=x-10

    y=\frac{x-10}{2}

    y=\frac{x-5}{2}

    I understand that they must have used the common denominator. I thought when you divide one part by the common denominator, you had to divide the other part also, so how do they know that 2 will go into x?

    Marilyn
    Last edited by Meggomumsie; September 30th 2009 at 01:19 AM. Reason: Forgot to say to change the subject
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  2. #2
    MHF Contributor
    Grandad's Avatar
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    South Coast of England
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    Hello Marilyn
    Quote Originally Posted by Meggomumsie View Post
    Hi, I'm really pleased to say that I have learned lots from your answers, thank you again. I do not understand why the last step is taken in the question below. (Change the subject of the formula to y)

    x-2y=10

    x-10=2y

    2y=x-10

    y=\frac{x-10}{2}

    y=\frac{x-5}{2}

    I understand that they must have used the common denominator. I thought when you divide one part by the common denominator, you had to divide the other part also, so how do they know that 2 will go into x?

    Marilyn
    You're right to query it - the last step is incorrect! The answer is

    y=\frac{x-10}{2}

    and you can't simplify it any further!

    Grandad
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  3. #3
    Junior Member
    Joined
    Aug 2009
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    Derbyshire, central UK
    Posts
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    Ooooh!! I'm learning!! I find the books are often wrong and as long as I can be told the correct answer, it helps because it makes me think. Thank you.
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