（x-400)/y=0.7 x=? y=? 请帮我 告诉我计算过程 谢谢

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 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+x+y+y+y=56 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 已知:3x-5y=0,求x/y,x-y/y,x-y/x 推倒公式:(x+y)(x*x-3x+y*y)=x*x*x+y*y*y怎么做. D：y>=x,y y'/y=1/x x+y=5; x^y+y^x=17 X+Y=40,(X*Y)/(X+Y)=7.5 因式分解下列式子：x(x-y)+y(y-x)=?