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    How do you know when you must add or subtract in a simultaneous equation?

    e.g. 3x+10y=7
    x - 4y = 6
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    (Original post by Halimah_1)
    How do you know when you must add or subtract in a simultaneous equation?

    e.g. 3x+10y=7
    x - 4y = 6
    same sign subtract, different sign add.

    eg

    3x+10y=7
    x-4y=6

    3x+10y=7
    3x-12y= 18

    in this case the x values r the same and r both positive so subtract them.
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    basically you get either the y or x to be equal so in this case you can do
    3x+10y=7 x2
    then
    x-4y=6 x5

    so you get

    6x+20y=14
    and
    5x-20y=30

    the y are both 20 so you have to add them because 20 + -20 = 20-20 = 0
    they cancel out so you minus the x and the number after the equal signs aswell
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    If the signs are different, you add.
    If the signs are the same you subtract.

    So in this example, if you were getting rid of x you would subtract, but if you were getting rid of y, you would add
 
 
 
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