解由(1-x)^2-4y(x-y-1)
=(1-x)^2+4y(1-x+y)
=(1-x)^2+4y(1-x）+4y^2
=[(1-x)+2y]^2
≥0
即(1-x)^2-4y(x-y-1)≥0
故
(1-x)^2≥4y(x-y-1)