One way is to first approximatef_X ~= (f(X+dX,Y) - f(X-dX,Y))/(2 dX)and thenf_XY = (f_X)_Y ~= [f_X(X,Y+dY) - f_X(X,Y-dY)]/(2 dY)~= [(f(X+dX,Y+dY) - f(X-dX,Y+dY))/(2 dX) - (f(X+dX,Y-dY) - f(X-dX,Y-dY))/(2 dX)]/(2 dY)= [f(X+dX,Y+dY) - f(X-dX,Y+dY) - f(X+dX,Y-dY) + f(X-dX,Y-dY)]/(4 dX dY).