Based on the improved Fermi's method, Clausius equality and inequality is derived directly from Kelvin's statement of the second law of thermodynamics, and then the theorem of entropy is introduced. Carnot's principle is not used in the derivation. 改进费米方法后,无须卡诺定理,即可由热力学第二定律开尔文表述直接推导克劳修斯等式、不等式,从而得到熵和熵增加原理。