y-x=3 7x+5y=-9

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y-x=37x+5y=-9y-x=37x+5y=-9y-x=37x+5y=-9y-x=3(1)7x+5y=-9(2)(1)*7+(2)12y=12y=1x=-2y-x=35y-5x=157x+5y=-

y-x=3 7x+5y=-9
y-x=3 7x+5y=-9

y-x=3 7x+5y=-9
y-x=3 (1)
7x+5y=-9(2)
(1)*7+(2)
12y=12
y=1
x=-2

y-x=3 5y-5x=15
7x+5y=-9 两式相减 -12x=24 x=-2 y=1

y-x=3(1)
7x+5y=-9 (2)
由(1)×7得
7y-7x=21(3)
由(3)+(2)得
12y=12
y=1
把y=1带入(1)
得x=-2

y-x=3
7x+5y=-9
y-x=3
y=3+x(带入)
7x+5(3+x)=-9
7x+15+5x=-9
12x+15=-9
12x=-9-15
12x=-24
x=-24/12
x=-2