证明组合恒等式,一般是利用组合数的性质、数学归纳法、二项式定理等,通过一些适当的计算或化简来完成.但是,很多组合恒等式,也可直接利用组合数的意义来证明.即构造一个组合问题的模型,把等式两边看成同一组合问题的两种计算方法,由结论的唯一性,即可证明组合恒等式.例1证明:C Proof of the identity of the portfolio, the general is the use of the nature of the number of combinations, mathematical induction, binomial theorem, etc., by some appropriate calculation or reduction to complete, but many combinations of identities can also be directly used to prove the significance of the combination number. That is, we construct a model of combinatorial problems and regard both sides of the equation as two computational methods of the same combinatorial problem. From the uniqueness of the conclusion, we can prove the combinatorial identity. Example 1 proves that C