lim(x→∞)x[√(1+x^2)-x]=lim(x→∞){x(1+x^2-x^2)/[√(1+x^2)+x]}=lim(x→∞){x/[√(1+x^2)+x]}=lim(x→∞){1/[√(1+1/x^2)+1]}=1/[√(1+0)+1]=1/2
嗯