Question

已知x2+y2+4x-8y+20=0先化简再求值

Answer 1

x2+y2+4x-8y+20=0

(x+2)^2+(y-4)^2=0

所以x=-2 y=4

(x-y)2(x+y)2-(x2+y2)2

=(x^2-y^2)^2-(x^2+y^2)^2

=2x^2*(-2y^2)

=-4*(xy)^2

=-4*(-8)^2

=-256