已知x + y = -1,x y = -2,求代数式x3 - y3的值

x+y = -1 (1)
xy =-2(2)
x-2/x =-1
x^2+x-2=0
(x+2)(x-1)=0
x=1 or -2
case 1: when x=1 ,y= -2
x^3- y^3 = 1+8=9
when x=-2 ,y= 1
x^3-y^3 = -8 -1 =-9
ie x^3- y^3 = 9 or -9