(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
