lim (x->+00) ((x - a)^(1/2) - x^(1/2)) = lim (x->+00) ((x - a)^(1/2) - x^(1/2)) * ((x - a)^(1/2) + x^(1/2))/((x - a)^(1/2) + x^(1/2)) = lim (x->+00) (x - a - x)/((x - a)^(1/2) + x^(1/2)) = 0.