lim x→0 y→0 2xy/根号下1+xy 然后-1
=lim x→0 y→0 2xy[√(1+xy)+1]/[√(1+xy)-1][√(1+xy)+1]
=lim x→0 y→0 2xy[√(1+xy)+1]/xy
=lim x→0 y→0 2[√(1+xy)+1]
=4