f(x)=(x^2-3x+1)/(x+1)
=[(x^2+2x+1)-5(x+1)+5]/(x+1)
=[(x+1)^2-5(x+1)+5]/(x+1)
=(x+1)+5/(x+1)-5
当x>-1时,x+1>0,
(x+1)+5/(x+1)≥2√5,(x+1)+5/(x+1)-5≥2√5-5.
当x