lim(√(n+3)-√n),n趋向∞,求极限
=lim (√(n+3)-√n)/(√(n+3)+√n)/(√(n+3)+√n)
=lim 3/(√(n+3)+√n)
=lim (3/√n)/[√(1+3/n)+1]
=0/2
=0