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# simultaneous equations Help

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1. 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
__________________
9x+12y=15
8x-12y=36

What's next?
2. If the y terms have the same sign then subtract and if they have a different sign then add
3. 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
4. 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:
2x-3y=9
2(3)-3y=9

rearrange the equation to find y:
2(3)-3y=9
6 - 3y = 9
-3y = 3
-y = 1
y = -1

so x = 3 and y = -1

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