1.xy-3x=40;x=5

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1.xy-3x=40;x=5
2.7x-xy=-18;x=-6
3.3xy-28=16x;x=4
4.9y+6xy=30;x=-6
5.y-2xy=15;x=-1
6.4x+7y+5xy=0;x=1

1.xy-3x=40
x(y-3)=40
y-3=40/x
代入x=5
y-3=40/5
y-3=8
y=8+3
y=11
2.7x-xy=-18
x(7-y)=-18
7-y=-18/x
代入x=-6
7-y=-18/(-6)
7-y=3
-y=3-7
-y=-4
y=4
3.3xy-28=16x
代入x=4
3*4y-28=16*4
12y-28=64
12y=64+28
12y=92
y=23/3（分数）
4.9y+6xy=30
代入x=-6
9y+6*(-6)y=30
9y-36y=30
y(9-36)=30
-27y=30
y=30/(-27)
y=-(10/9)
5.y-2xy=15
y(1-2x)=15
代入x=-1
y[1-2*(-1)]=15
3y=15
y=5
6.4x+7y+5xy=0
代入x=1
4*1+7y+5*1*y=0
4+7y+5y=0
4+12y=0
12y=-4
y=-(1/3)

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