√正确
f'(x)={[1+e^(1/x)]-x*(e^(1/x))'} /[1+e^(1/x)]²
={[1+e^(1/x)]-x*(e^(1/x))*（-1/x²)} /[1+e^(1/x)]²
=[x+(x+1)e^(1/x)]/{x[(1+e^(1/x)]²}
