when doing a simultaneous equation, how do you know how to plus it or minus it? eg. 1. x+4y=7 2. 2x-3y=4 do you plus or minus it? Also, which one do you change to make it equal? either x and 2x ( times x by 2 to make them both 2x or 4y by 3 and 3y by 4 to make them both equal 12y)|||Don't do either, that method always confused me. Instead, make one of them x= or y=, then just substitute it into the other. You'll get taught to do that when you use simultaneous equations with x^2 etc in anyway. So with this one, make the top equation x = 7 - 4y If x = 7 - 4y, then you can replace all x's with (7 - 4y), so substitute that in to the bottom equation: 2(7 - 4y) - 3y = 4 Now just simplify to find what y equals: 14 - 8y - 3y = 4 14 = 4 + 11y 11y = 11 y = 1 Now you can substitute y into the original equation as normal: x + 4x1 = 7 x + 4 = 7 x = 3 Done! Hope that helped|||The two basic methods are SUBSTITUTION and ELIMINATION, and in either case you gan 'go for' X or Y. Just pick the method that --- based on the specific problem --- seems easiest. Substitution; x+4y=7 becomes x=7-4y and plug that into the other equation to get 2(7-4y)-3y=4 Elimination; x+4y=7 times 2 first. 2x + 8y = 14 2x - 3y = 4 ---------------------- subtract 11 y = 10|||It doesn't matter which eq. you choose. x+4y=7 2x-3y=4 4x+16y=28 4x-6y=8 I subtract the second from the first. 22y=20 y=10/11 x+4y=7 2x-3y=4 2x-3y=4 x+4y=7 2x-3y=4 2x+8y=14 -11y=-10 y=10/11 I subtract the first from the second.|||X =7-4y鈥︹€?call this鈥?.equation 1 why? because is simpler than to say, x=(3y+4)/2. just change the simplest equation and you will never go wrong.

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