初一的很简单的二元一次方程,好的追加100

x+y=450 (1)
(1-0.6)x=(1-0.4)y-30.
0.4x=0.6y-30
0.4x-0.6y=-30
2x-3y=-150 (2)
(1)*3+(2)
3x+3y+2x-3y=1350-150
5x=1200
x=240
y=450-x=210
2x+2y=1 (1)
6x-6y=1 (2)
(1)-(2)
-4x+8y=0
x=2y,代入（1）
2*2y+2y=1
6y=1
y=1/6
x=2y=1/3
2x+y=15 (1)
x+2y=2.(2)
(1)+(2)
3x+3y=17
x+y=17/3 (3)
(1)-(3)
x=28/3
y=17/3-x=-11/3
5x+y=3 (1)
x+5y=2 (2)
(1)+(2)
6x+6y=5
x+y=5/6 (3)
(1)-(3)
4x=13/6
x=13/24
y=5/6-x=7/24

x+y=450...............1,
(1-0.6)x=(1-0.4)y-30
0.4x=0.6y-30
4x-6y=-300................2
1式*4-2式
10y=2100
y=210
x=240
2.
2x+2y=1.............1
6x-6y=1 ..............

1.{{x -> 240., y -> 210.}}
2.{{x -> 1/3, y -> 1/6}}
3.{{x -> 28/3, y -> -11/3}}
4.{{x -> 13/24, y -> 7/24}}

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