>>469

(1/2)*(f(x,y+s')-f(x,y))/(-s') -(1/2)*(f(x,y+s)-f(x,y))/s
=-(1/2)*(f(x,y+s')-f(x,y))/s' -(1/2)*f(x,y+s)-f(x,y))/s
→-(1/2)∂f/∂y-(1/2)∂f/fy=-∂f/∂y