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simultaneous equations Help

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    • Thread Starter

    Normally on a maths exam paper I get 1 or 2 marks out of 4 for this because I can't tell the difference to add or take away which expressions e.g:

    November 2013 1H paper:

    3x+4y=5 ........ x3
    2x-3y=9 .............x4

    What's next?

    If the y terms have the same sign then subtract and if they have a different sign then add

    Cancel y terms. 9x plus 8x makes 17x and 15 plus 36 makes 51.

    51/17=x x= 3

    Sub into equation 1 or 2

    because, in the second equation, the sign before the 12y is minus, you add them together.
    9x+12y= 15
    + 8x-12y= 36
    17x = 51
    x = 3

    substitute x into one of the original equations:

    rearrange the equation to find y:
    6 - 3y = 9
    -3y = 3
    -y = 1
    y = -1

    so x = 3 and y = -1
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