A must use the mathematical symbol "? One ". It is a logical operator, which means "non-A", that is, the true value in a logical expression can be turned into a false value, or the false value can be turned into a true value. For example, there is an expression A∧B (for A and B). What is it using? You'll get it after the operation? A∧B- the expression here? A is the reverse operation of the original logical expression. Therefore, from the above, we can also see that a in mathematics is actually the reverse operation of the original logical expression.