lim(x→∞) x[√(x²+1)-x]
(上下同时乘以√(x²+1)+x)
=lim(x→∞) x/[√(x²+1)+x]
(上下同时除以x)
=lim(x→∞) 1/[√(1+1/x²)+1]
=1/2