x^2-4xy+4y^2-x+2y-6=(x-2y)^2-(x-2y)-6(理由,为什么这么做)什么叫十字相乘啊？不好意思

2x^+xy-y^-4x+5y-6=? 1-2/x+y=xy,4y+6x=xy.求xy的解? 因式分解 x-xy+3y-3x 2x+xy-y-4x+5y-6 诺x+y=6,xy=4 求(5x-4y-3xy)-(8x-y+2xy) x^+2xy+y^+4=16 x^-2xy+y^=16 求(3xy+6y^)+|x^-(2xy+2y^-3x^)|的值 化简求值x-4y/2x-7xy+3y×x-2xy-3y/2x+3xy-2y÷3x-6y/4x-4xy+y,其中：x=1999,y=-1 2(x*x-3xy-2y*y+2)- =-4y*y-6xy+3x 已知x*x-4xy+4y*y=0 求[2x(x+y)-y(x+y)]/(4x*x-4xy+y*y)的值? 因式分解:12xy^2(y-x)+xy^3(x-y)+4x^2y(x-y) （-3x^y+2xy)-( )=4x^+xy 4xy-[(x²+5xy-y²)-(3xy-1/2y)]x=3,y=-6 已知1/x-1/y=2,求3x+7xy-3y/2x-3xy-2y答案是1/x-1/y=2(y-x)/xy=2 得：x-y=-2xy(3x+7xy-3y)/(2x-3xy-2y)=[3(x-y)+7xy]/[2(x-y)-3xy]=(-6xy+7xy)/(-4xy-3xy)=xy/(-7xy)=-1/7 谁能帮我解释一下=[3(x-y)+7xy]/[2(x-y)-3xy]=(-6xy+7xy)/(-4xy-3xy) 是怎 (x+y)(x+y+2xy)+(xy+1)(xy-1)(x+y)(x-y)+4(y-1) 计算：[(x-y)^2-(x+y)^2]/(-4xy)= 2X^+XY-Y^-4X+5Y-6 若x+y=4,x-y=6,求xy(y^2+y)-y^2(xy+2x)-3xy的值. 已知；x+y=4,x-y的绝对值=6,求xy(y的平方+y)-y的平方（xy+2x）-3xy先化简再求值 已知x+y=4,x-y=6,化简xy（y²+y）-y²（xy+2x）-3xy