X趋∞：lim(2x²+5)/(x+1)•[√(x²+1)-√(x²-1)]=2lim(2x²+5)/{[√(x²+1)+√(x²-1)](x+1)}=2lim(2x+5/x²)/{[√(1+1/x²)+√(1-1/x²)](1+1/x)}=2
