30x=0.8y 40(x-1)=y+25用加减消元法来解,

x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 （x-y)(x+y)-(x+y)^2+2y(y-x),其中x=1,y=3. 2y(x+二分之一y)-[(x+y)(x-y)+2y(y+x)],其中|x-1|=2 已知x+y=a,2x-y=-2a,求[（x/y-y/x)/(x+y)-x(1/x-1/y)]/[(x+1)/y]的值 若2/x-1/y=3,求[y/x-y/x-y(x-y/x-x+y)]/x-2y/x的值 x+y/3+x-y/2=6 4(x+y)-2(x-y)=40 {6/x+y+10/x-y=30 6/x+y-10/x-y=-1 x+2y=2x+y+1=7x-y 求：2x-y? 设x,y满足约束条件x+y>=1,x-y>=-1,2x-y 设x,y满足约束条件x+y>=1,x-y>=-1,2x-y 二元一次方程 ：2(x+y)-(x-y)=3 (x+y)-2(x-y)=1 (3x-y)^2+(3x+y)(3x-y),x=1,y=-2 2(x+y)-3(x-y)=1 6(x+y)+(x-y)=51 {3(x+y)-4(x-y)=4 {x+y/2 + x-y/6=1 {6(x-y)-7(x+y)=21 {2(x-y)-5(x+y)=-1 变量x,y满足约束条件,x+y>=3,x-y>=-1,2x-y (x-y)^2+(x+y)(x-y) 其中 X =3 Y=-1 1=x+y 2=20x+30y