Theorem 3.6.2. Let f be a function of two variables with domain D C R2 an open set. Let u = (u1, u2) be a unit vector. If f is differentiable at (a, b) E D, then Daf(a, b) exists and Daf(a, b) = Vf(a, b) . u. Theorem 3.2.13. Let f be a function of two variables with domain D C R2 an open set. If f is differentiable at (a, b) E D, then the directional derivative of f at (a, b) depends continuously on the direction. That is, lim Def(a, b) = Deof(a, b) 0-+00 for every do E [0, 27)