(x+1)/20=(y+1)/21=(x+y)/17

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 y=[x-1], 1.已知1/x+1/y=-1/x+y,则y/x+x/y=? {3(x+y)+3(y+x)=1,3(x+y)+4(y-x)=-1 ｛4/(x+y)+6/(x-y)=3 ｛9/(x-y)-1/(x+y)=1 (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 若x-y=1,求代数式x(x-y)+y(y-x)+2013的值 已知4x=9y求(1)x+y/y (2)y-x/2x y'=(x-y+1)/(x+y-3)通解 x2-x-y2-y 解法：=(x2-y2)-(x+y) =(x+y)(x-y)-(x+y) =(x+y)(x-y-1) 1/(x+y)-1/(x+y)=3/20 2/(x+y)+3/(x-y)=19/20 这是