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Math Help - Rearranging simulataneous equation into y = mx + c

  1. #1
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    Rearranging simulataneous equation into y = mx + c

    I haven't done maths in years as I thought I wouldn't ever do it again after school but it has cropped up in university and I've forgotten it all.
    So...

    I need to plot the following on a graph in Excel
    3x + y - 6 = 0 (i)
    y - 3x + 2 = 0 (ii)

    I need to rearrange this, right?
    I was thinking (this is probably wrong)...

    (i): y = -3x - 6
    (ii): y = 3x + 2

    I would appreciate any help and also how to rearrange equations like this. I understand you have to make y the subject here and that you change the sign of something...but I don't know what. :s

    Thanks.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by Jamie88 View Post
    I haven't done maths in years as I thought I wouldn't ever do it again after school but it has cropped up in university and I've forgotten it all.
    So...

    I need to plot the following on a graph in Excel
    3x + y - 6 = 0 (i)
    y - 3x + 2 = 0 (ii)

    I need to rearrange this, right?
    I was thinking (this is probably wrong)...

    (i): y = -3x - 6
    (ii): y = 3x + 2

    I would appreciate any help and also how to rearrange equations like this. I understand you have to make y the subject here and that you change the sign of something...but I don't know what. :s

    Thanks.
    The secret in elementary algebra is what you do to one side of an equation you also do to the other side.

    In (i) you start with:

    3x + y - 6 = 0

    Now subtract 3x from both sides to get:

    3x +y - 6 -3x = -3x

    or:

    y - 6 = -3x

    Now add 6 to both sides:

    y - 6 + 6 = -3x + 6

    or:

    y = -3x + 6

    RonL
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  3. #3
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    Quote Originally Posted by CaptainBlack View Post
    The secret in elementary algebra is what you do to one side of an equation you also do to the other side.

    In (i) you start with:

    3x + y - 6 = 0

    Now subtract 3x from both sides to get:

    3x +y - 6 -3x = -3x

    or:

    y - 6 = -3x

    Now add 6 to both sides:

    y - 6 + 6 = -3x + 6

    or:

    y = -3x + 6

    RonL
    Okay so this applies to (ii) as well, yeah?
    (ii) y - 3x + 2 = 0

    Do I add the -3x here or subtract it as it is a minus...
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  4. #4
    Grand Panjandrum
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    Quote Originally Posted by Jamie88 View Post
    Okay so this applies to (ii) as well, yeah?
    (ii) y - 3x + 2 = 0

    Do I add the -3x here or subtract it as it is a minus...
    You add 3x as (-3x)+3x=0 and the idea is to remove this term from the left hand side, so:

    y - 3x + 2 + 3x = 3x

    or:

    y +(-3x+3x) +2 = 3x

    so:

    y +2 = 3x.

    (adding 3x is the same thing as subtracting -3x that is 3x=-(-3x))

    RonL
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  5. #5
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    So...

    y = 3x - 2

    ?
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  6. #6
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    Quote Originally Posted by Jamie88 View Post
    So...

    y = 3x - 2

    ?
    Yes.

    -Dan
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