Details are as follows:
C=C
C=5×4×3/3×2× 1= 10
C=C=C=5×4/2× 1= 10
So the permutation combination c is equal to C.
Extended data:
Related difficulties
1. Abstracting several concrete mathematical models from different practical problems requires strong abstract thinking ability;
2. Restrictive conditions are sometimes obscure, which requires us to accurately understand the key words in the question (especially logical related words and quantifiers);
3. The calculation method is simple and has little connection with the old knowledge, but it needs a lot of thinking when choosing the correct and reasonable calculation scheme;
4, the calculation scheme is correct, often can't use intuitive method to test, requires us to understand the concept, principle, and have strong analytical ability.