X+Y=158X+4Y=72X+Y=15Y=15-X代入8X+4Y=728X+4(15-X)=728X+60-4X=724X=12X=3Y=15-X=124X=12这一步怎么解出来的。说下

x-y/x-x+y/y-(x+y)(x-y)/y² y/x=2 化简 x / y(x+y) - y / x(x+y) = 2（x+y)-3(x-y)=3 ,4(x+y)+3(x-y)=15 x(x+y)(x-y)-y(y+x)(y-x)=(x-y)( )填空 (x-y) (x-2y)+(x+2y) (x-3y)-2(x-3y) (x-4y)= 7x:4y,x:y=( ):( (x-4y)(2x+y)= 5x-4y(y-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=? y=x(x x+x+x+y=60 .x-y=4 x+x+x+x+y+y+y=56 4x方y/(x-2y)(x+2y)怎么算[x方/x-2y]-4x方y/(x-2y)(x+2y)=? 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) x-y/x+y=(x-y)²/x²-y²（x≠y) （x+y-5)*（x+y-5)+（x-y+3)=0 x*x-y*y