若X+Y=3,XY=2,求x的平方—y的平方—2xy
问题描述：
若X+Y=3,XY=2,求x的平方—y的平方—2xy
答
（x+y)^2=x^2+y^2+2xy=（x-y)^2+4xy=9 (x-y)^2=9-8=1 x-y=±1 x^2-y^2-2xy =(x+y)(x-y)-2*2 =3(x-y)-4 =-1或-7
若X+Y=3,XY=2,求x的平方—y的平方—2xy
（x+y)^2=x^2+y^2+2xy=（x-y)^2+4xy=9 (x-y)^2=9-8=1 x-y=±1 x^2-y^2-2xy =(x+y)(x-y)-2*2 =3(x-y)-4 =-1或-7