｛x+y=100 0.7x+0.8y=74

｛x+y=100 0.7x+0.8y=74 X+Y=7(100X+Y)-(10Y+X)=(10Y+X)-(10X+Y) x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 化简 x / y(x+y) - y / x(x+y) = 7x:4y,x:y=( ):( x(x+y)(x-y)-y(y+x)(y-x)=(x-y)( )填空 x+y=22 2.5x+7y=100 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=? y=x(x 解方程组(x+y)(x-2y)=-5 (x-y)(x+2y)=7 {6(x-y)-7(x+y)=21 {2(x-y)-5(x+y)=-1 x+2y=2x+y+1=7x-y 求：2x-y? 当x+y=2时7(x+y)一(x+y)+(x+y)=? x^2+y^2=15,x+y=7,x＞y,求x-y 若(x-y)/(x+y)=3 则(x+2y)/(6x-7y)等于 x+x+x+x+y+y+y=56 (x-y)(x-2y)+(x-2y)(x-7y),其中X=4,Y=1.5