｛2x-5y=7 ｛2x+3y=－1

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 已知x-y/x+y=3,求代数式5(x-y)/x+y-x+y/2(x-y) 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[(3x+4y)^2-(2x+y)(2x-y)+(-x+y)(5x-y)]除以-2y,其中x=-1,y=1 7x-3y/5=5x-y/3-x+y/2 x-1/5=y+1/3 { x-y/7-x+y/10=1/2 {2(x-y)+5(x+y)=3 已知x=1/3,y=-1/2,求代数式x-(x+y)+(x+2y)-(x+3y)+(x+4y)-(x+5y)+...-( 若2x-3y+4=0则x（x*x-1)+x(5-x*x)-6y+7 （2x+y-1)(2x-y+1)-(3x+y)(3x-y),其中x=5分之1, 解方程组(x+y)(x-2y)=-5 (x-y)(x+2y)=7 (3x-y)^2+(3x+y)(3x-y),x=1,y=-2 6(x-y)-2(x+y)=14 3(x-y)+(x+y)=5