y=a+16/(-(b-a/2)^2+a^2/4)
>=a+16/(a^2/4)
=a/2+a/2+64/(a^2)
>=6*(2的立方根) // [a+b+c>=3*(abc的立方根)]
当且仅当:a=4*(2的立方根);b=2*(2的立方根);