{x+y=420,30%x+40%y=160×80%解方程

{6/x+y+10/x-y=30 6/x+y-10/x-y=-1 （1）x+y=420,30%x+40%y=160×80% （2）x-y/7-x+y/6=1/2,2(x-y)-5(x+y)+1=0,求方程组解嗷嗷嗷! x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 y'/y=1/x X+Y=40,(X*Y)/(X+Y)=7.5 化简 x / y(x+y) - y / x(x+y) = y'-(1/x)y=xe^-x x(x+y)(x-y)-y(y+x)(y-x)=(x-y)( )填空 y(x+y)+(x-y)²-（x+y）（x-y）,其中x=-1/3,y=3 （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 x+x=y+30 x>=1,x-y y=x-1(x 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 x.y都是自然数,且x(x-u)-y(y-x)+12,求x.y得值快1且x(x-y)-y(y-x)=12 若2/x-1/y=3,求[y/x-y/x-y(x-y/x-x+y)]/x-2y/x的值 x+y=450;40%x+30=60%y