｛x+y=490 ｛0.9x+0.9y=413 -

｛x+y=490 ｛0.9x+0.9y=413 - x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 化简 x / y(x+y) - y / x(x+y) = x(x+y)(x-y)-y(y+x)(y-x)=(x-y)( )填空 0.9x-y=215 0.8x-y=-125 0.9Y=X+2150;0.8Y=X-1250 化简:6(0.9y-x)=9(y-30-x) x+x+y+y+y=21 ,x+x+y+y+y+y+y=27 ,x ,y=? x+x+y+y+y=21 ,x+x+y+y+y+y+y=27 ,x ,y=? 解方程0.9x-y=215,0.8x-y=-125x=,y= 0.9x=1.1y y+x=360 x=?y=? y=x(x 求解方程组X—Y=630 1.2X+0.9Y=1.12X+1.12Y x + y =498；0.7x + 0.9y =386.x,y是多少 x+x+x+x+y+y+y=56 A:0.22=X*1.3*(Y-X) B:0.16=X*0.9*(Y-X)二元一次方程 x+x+x+y+y=6.4 y+y+y+x+x=7.1 怎么算? 二元函数f(x,y)=x+y/x-y,求f(y/x,x/y)