唔識計聯立方程呀

2(x+y)-3x=4
x = 2y -4 --(1)

3(x-y)+5y=12
3x +2y= 12 --(2)

Sub (1) into (2)

3(2y -4) + 2y = 12
8y = 12 + 12
y =3
x = 2

2(x+y)-3x=4
x = 2y -4 --------------(1)
3(x-y)+5y=12
3x +2y= 12 ------------(2)

Put (1) into (2)

3(2y -4) + 2y = 12
8y = 12 + 12
y =3
x = 2

2(x+y)-3x=4 ----- (1)
3(x-y)+5y=12 ----- (2)

(1):
2(x+y)-3x=4
2x+2y-3x=4
-x+2y=4
x=2y-4 ----- (3)

put (3) into (2):
3(x-y)+5y=12
3{(2y-4)-y}+5y=12
3(y-4)+5y=12
3y-12+5y=12
8y=24
y=3

put y=3 into (3):
x=2y-4
x=2(3)-4
x=6-4
x=2


可有更快的方法,但最重要,最基本的技巧是選一個未知數(X或Y),
利用移項把它搬到方程式的一方,再把它套入另一條方程式中,
以代入這條方程式,來消去2個未知數方程式的其中一個.