f(x,y)=x/(x+y)
f'x(x,y)=(x+y-x)/(x+y)^2=y/(x+y)^2,则f'x(1,2)=2/(1+2)^2=2/9
f'y(x,y)=-x/(x+y)^2,则f'y(x,y)=-1/(1+2)^2=-1/9
有不懂的再追问.