若x-2y

若(x-y)/(x+y)=3 则(x+2y)/(6x-7y)等于 设x>1,y>0,若x^y+x^-y=2根号2,则x^y-x^-y等于 若(x*x+y*y)(x*x+y*y)-4x*x*y*y=0,求代数式(x*x+5xy+y*y)/(x*x+2xy+y* 若|x+2y-1|+y²+4y+4=0,求（2x-y）²-2（2x-y)(x+2y)+(x+2y)& 若变量x,y满足约束条件{x>=-1;y>=x;3x+2y 若|x+y-1|+（x-y-2)²=0,求代数式（x+2y)(x-2y）-（2x-y)(-y-2x)的值. 若x=-1/4,能否确定代数式(2x-y)(2x+y)+(2x-y)(y-4x)+2y(y-3x)的值? 若X=负四分之一,能否确定代数式(2X-Y)(2X+Y)+(2X-Y)(Y-4X)+2Y(Y-3X)的值? 若3x-5y=0,则x+y/x=（）,x-y/y=（）,2x-3y/2x+3y=( ) 若x-y/x+y=5,求x-y/2x+2y+5x+5y/3x-3y的值 若全集I={(x,y)|x,y∈R},集合M={(x,y)|(y-3)/(x-2)},N{(x,y)|y=x=1},则( 1、x(x-y)(x+y)-x(x+y)^2