y(u(x+h))-y(u(x))/h
u(x+h)=u(x)+du/dx(x)h
y(u(x+h))-y(u(x))=((y(u(x)+du/dx(x)h)-y(u(x)))/du/dx(x)h)*(du/dx(x))
=(dy/du)(du/dx)