Proving continuity of binary function by differentiability How to prove Just one point is enough
It's very easy for you to deform the limit formula. What we can say is that (I use DX, Dy to express the increment of X, y), there are numbers a and B such that LIM (DX, Dy tends to zero) [f (x0 + DX, Y0 + dy) - F (x0, Y0) - ADX - bdy] / sqrt [(DX) ^ 2 + (Dy) ^ 2] = 0, because f (x0 +...)