解以下各聯立二元一次方程

1. y = 2x ---(1)
3x - y = 3 ---(2)
Put (1) into (2),
3x - 2x = 3
x = 3
Put x = 3 into (1),
y = 2(3) = 6

2. y = x+2 ---(1)
2x - y = 0 ---(2)
Put (1) into (2),
2x - (x+2) = 0
x - 2 = 0
x = 2
Put x = 2 into(1)
y = 2+2 = 4

3. y = 1-x ---(1)
4x+y = 10 ---(2)
Put (1) into (2),
4x + (1 - x) = 10
3x = 9
x = 3
Put x = 3 into (1)
y = 1 - 3 = -2

4. y = 2x - 3 ---(1)
2y - x = 3 ---(2)
Put (1) into (2),
2(2x - 3) - x = 3
4x - 6 - x = 3
3x = 9
x = 3
Put x = 3 into (1)
y = 2(3) - 3 = 3