x^2+xy-y^2+9=(x^2-2xy)+(3xy-y^2)+9=7+4+9=20为什么?

(x-2xy)*(-xy+2y*y)-(3x*x-2xy)(x-9xy+6y*y)答案应为-2x-x*x-3x， （x^2+xy/x-y)/（xy/x-y)计算 已知x-xy=8,xy-y=-9,求x+y-2xy的值 (-x-y)(x^2+xy+y^2) 化简x^-y^/x^-2xy+y^ 2*(x-y)²(y-x)(xy) （-3x^y+2xy)-( )=4x^+xy (XY-X^2)除以X-Y/XY=? (x+y)/x /(x-(2xy-y^2)/x) 因式分解 化简：x-y/x+3y÷x^-y^2/x^2+6xy+9y^2-(xy/x+y) f(xy,x-y)=x^2+y^2+xy;f(x,y)/∂x;f(x,y)/∂y x*x+xy=3,xy+y*y=2,则x*x+2xy+y*y=________? (x^3-2x^2+xy^2)/(x-y)+(x+y)(y-x)=? 代数 x^-y^/ x^-(y-z)^ 除以 x^+2xy+y^/(x-y)^-z^ 乘 x^+xy-xz/x^-xy X-Y/2X+2Y XY/X+Y² 因式分解：x×x-xy-2y×y-x-y （x*x-2xy）*9x*x-(9xy*y*y-12x*x*x*x*y*y)/3xy,其中x=1,y=-1 化简 帮个忙, 已知x*x-4xy+4y*y=0 求[2x(x+y)-y(x+y)]/(4x*x-4xy+y*y)的值?