f(x,y) = f(a,b) + df(a,b)/dx[x - a] + df(a,b)/dy[y - b] + d^2f(a,b)/dx^2[x-a]^2/2 + d^2f(a,b)/dy^2[y-b]^2/2 + d^2f(a,b)/[dxdy][x-a][y-b] + h.
其中,h为余项.
当f(x,y)2阶导数连续,x->a,y->b时,h是[(x-a)(y-b)]的高阶无穷小量.