This is the knowledge about permutation and combination. 4 in the lower right corner represents the total number of 4. M in the upper right corner must be an integer greater than or equal to zero but not greater than the number in the lower right corner (this is called 4), but it is a natural number, A( 1/4)=4, which means four things. There are several ways to take something out. A(2/4) means 4. A is arrangement, and different orders also belong to different situations. For example, four people, Party A, Party B and Party D, take two people out, and Party A and Party B can be arranged in Party A and Party B. There are two situations. C is the symbol of combination, which is different from arrangement, for example, C(2/4)=4x3/2/ 1=6, and the combination is out of order, for example, four people, Party A and Party B, got it. Typing is not easy to adopt! I don't understand.