7x2-12xy+5y2=0,（7x-5y）（x-y）=0 这一步到这一步是为什么?

已知x2+y2+4x-2y+5=0,求x2+2xy+y2值 已知x2+y2 =7,xy=-2.求5x2 -3xy-4y2 -11xy-7x2 +2y2 已知x2+y2=7,xy=-2,求5x2—3xy—4y2—11xy—7x2+2y2的值 5x2-3xy-4y2-11xy+(-7x2)-(-2y2),其中x2+y2=7,xy=-2,求值 已知X2+y2=7,Xy=-2,求多项式5X2一3Xy-4y2-11Xy-7X2+2y2的值. 若3x2-4xy+y2=-3,2x2+3xy-2y2=8,求5x2-18xy+7y2的值. 1*{x+y+xy=7{x2+y2+xy=132*{x2+y2=10{x+xy+y=7 解方程组 x2 -2xy -y2 =1 2x2 -5xy -3y2 =0解方程组 x2 -2xy -y2 =12x2 -5xy -3y2 =0 x2-7xy+12y2=0(y不等于0)求x,y是平方 已知x2-7xy+12y2=0,求x/y的值 已知x+y=5,xy=7则x2+y2的值是什么 多项式x2-3xy+2y2-2加上( )等于5x2-7xy-3y2 解方程组 x2 -2xy -y2 =1 2x2 -5xy -3y2 =0 已知:x2+y2=7,xy=-2.求7x2-3xy-2y2-11xy-5x2+4y2已知:x2+y2=7,xy=-2.求7x2-3xy-2y2-11xy-5x2+4y2 已知x+y=5,xy=-12,求代数式x2-xy+y2 如果2/x=3/y,那么x2-2xy+3y2/7x2-y2? 已知x2+y2,x-y=5,求x2+y2和xy的值. X2+xy-12y2-2x+27y+5因式分解