已知X+Y=4,XY=-12那么X^2+Y^2= X^2-XY+Y^2= (X-Y)^2=
x^2+y^2=（x+y）^2-2xy=16-2*(-12)=40
X^2-XY+Y^2=(x-y)^2-3xy=16-(-12)=52
(x-y)^2=(x+y)^2-4xy=16-4（-12）=64