Ref : Differential과 Derivative의 차이는? 어떻게 이해해야 하나? ■ Differentials z = f(x, y). If △x and △y are the increments of x and y, then the increment of z is △z = f(x + △x, y + △y) - f(x, y) Definition: If z = f(x, y), then f is differentiable at (a, b) if △z can be expressed as, If z = f(x, y), then f is differentiable at (a, b) if the partial derivatives fx and fy exist and are continuous at (a, b) Fo..