这里^2指2次方
1/2为2分之1
y(x+y)-(2x+y)(x-y)-x^2
=xy+y^2-(2x^2-2xy+xy-y^2)-x^2
=xy+y^2-2x^2+xy+y^2-x^2
=2y^2+2xy-2x^2-x^2
=2y^2+2xy-3x^2
当x=-2,y=1/2时
y(x+y)-(2x+y)(x-y)-x^2
=2*(1/2)^2+2*(-2)*(1/2)-3(-2)^2
=1/2-2-12
=13.5=2y^2+2xy-3x^2当x=-2，y=1/2时=2y^2+2xy-3x^2=1/2-2-12 =-13.5