C first operate the upper and lower numbers, and then operate the next number. Mathematically, operation is a behavior, and a new quantity is obtained through possible combinations of known quantities. The essence of operation is the mapping between sets. Generally speaking, operation refers to algebraic operation, which is the correspondence in a set. For a pair of elements A and B taken out in sequence in set A, there is a unique third element C corresponding to it in set A, which is called the operation defined in set A..

From this operation, we can get two operations, that is, one of A and B is expected and C is known. The operation thus obtained is called the inverse operation of the original operation. For example, addition is the operation of finding a+b=c when A and B are known, then the operation of finding B when A and C are known, or the operation of finding A when B and C are known, is the inverse operation of addition, which is called subtraction.