(x,y) ->(0,0) => u = xy ->0
lim (x,y) ->(0,0) xy / [√(xy+1) - 1]
= lim u ->0 u / [√(u+1) - 1]
= lim u ->0 u *[√(u+1) + 1] / u
= lim u ->0 [√(u+1) + 1]
= 2