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Math Help - combining fractions

  1. #1
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    Unhappy combining fractions

    If You know how to Combine Like Fractions or know of a site where I can plug in problems and get tips and how to do them then PLEASE help me!

    9) -2(c-d) + (c-d) - 6(c-d)

    10) -3(4x + -2y) - 2(x+3y) -2(2x+6y)

    These are two problems I need hints on just to randomly pick!
    Last edited by demo; November 13th 2006 at 01:57 PM. Reason: the light blue color
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  2. #2
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by demo View Post
    If You know how to Combine Like Fractions or know of a site where I can plug in problems and get tips and how to do them then PLEASE help me!

    9) -2(c-d) + (c-d) - 6(c-d)

    10) -3(4x + -2y) - 2(x+3y) -2(2x+6y)


    These are two problems I need hints on just to randomly pick!
    First tip: Type your problems in a black font. It shows up much better on white. Really!

    Let's take the first term of the first problem:

    -2(c-d) = (-2)(c) + (-2)(-d) = -2c + 2d.

    This is called the distributive law and it always works like I did it above.

    So:
    9) -2(c - d) + (c - d) - 6(c - d)

    = (-2)(c) + (-2)(-d) + c - d + (-6)(c) + (-6)(-d)

    = -2c + 2d + c - d - 6c + 6d

    = -2c + c - 6c + 2d - d + 6d <-- I just rearranged the line above to group terms with c and d.

    Now we can reverse the distributive law. Look at the first 3 terms:
    -2c + c - 6c = (-2 + 1 - 6)c = -7c

    Likewise:
    2d - d + 6d = (2 - 1 + 6)d = 7d

    Putting it all together:
    -2(c - d) + (c - d) - 6(c - d) = -7c + 7d

    See if you can do the other one:
    -3(4x + -2y) - 2(x+3y) -2(2x+6y)

    I got -10x - 12y.

    -Dan
    Last edited by topsquark; November 13th 2006 at 01:52 PM. Reason: Added note
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  3. #3
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    Thanks...I'll try it!
    Last edited by demo; November 13th 2006 at 02:20 PM. Reason: trying
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  4. #4
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    so the answer for #9 is -7 + 7d ?

    and for 4(x + 5y) + 3(x + 6y) + 6(3x + 8y)
    I got 25x + 66y
    Last edited by demo; November 13th 2006 at 02:20 PM. Reason: got confused
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  5. #5
    Forum Admin topsquark's Avatar
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    Quote Originally Posted by demo View Post
    so the answer for #9 is -7 + 7d ?

    and for 4(x + 5y) + 3(x + 6y) + 6(3x + 8y)
    I got 25x + 66y
    The answer for 9 is -7c + 7d.

    4(x + 5y) + 3(x + 6y) + 6(3x + 8y)

    = 4x + 20y + 3x + 18y + 18x + 48y

    = 4x + 3x + 18x + 20y + 18y + 48y

    = (4 + 3 + 18)x + (20 + 18 + 48)y

    = 25x + 86y

    -Dan
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  6. #6
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    Talking Finally Done!

    damnit!
    I was so close to..well thanks!
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