y=x^2-x-1=(x-1/2)^2-5/4 (y>-5/4)
(x-1/2)^2=y+5/4
当x>=1/2时x-1/2=√(y+5/4),x=√(y+5/4)+1/2
当x1/2)
y=-√(x+5/4)+1/2 (y