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# Mathmatics GCSE Edexcel help!

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

e.g. 3x+10y=7
x - 4y = 6
2. (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.
3. 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
4. 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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