1/(x+y)-1/(x-y)=-3/20,2/(x+y)+3/(y-x)=-(1/5)急啊!

先化简再求值(x-y)(x+y)-(x-2y) 的完全平方+x(3x-5y)-(x-y)(x-2y),其中x=1/2 y 化简[(3x+4y)^2-(2x+y)(2x-y)+(-x+y)(5x-y)]除以-2y,其中x=-1,y=1 已知x=1/3,y=-1/2,求代数式x-(x+y)+(x+2y)-(x+3y)+(x+4y)-(x+5y)+...-( (3x-y)^2+(3x+y)(3x-y),x=1,y=-2 {3(x+y)-4(x-y)=4 {x+y/2 + x-y/6=1 2(x+y) 3x+3y=24 x+y/2x x y/2y= 1 （2x+y-1)(2x-y+1)-(3x+y)(3x-y),其中x=5分之1, 3X+5y=2 x+y=1 3x-2y=1 4x-5y x+y=3 x+2y=9 3/x+1=y y-x=3 y=2x-3 y-3x=1 2(x+1)-y=6 7x-5y=-9 化简求值(x+2y)^2-(2x+y)(2x-y)-5y(y-x) 其中 x=1/3 y= -1/2 {3(x+y)+3(y+x)=1,3(x+y)+4(y-x)=-1