y+9/z=3
y=3-9/z=(3z-9)/z
1/y=z/(3z-9)=z/[3(z-3)]
所以9/y=9z/[3(z-3)]=3z/(z-3)
代入x+9/y=3
x+3z/(z-3)=3
x=3-3z/(z-3)=(3z-9-3z)/(z-3)
x=-9/(z-3)
两边乘z-3
x(z-3)=-9
z-3=-9/x
z+9/x=3