命题

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对于两个命题，若是一个命题的条件核结论分别是另外一个命题的结论和条件，那么这两个命题叫作互逆命题，其中一个命题叫作原命题，另外一个命题叫作原命题的逆命题；

若是两个命题其中一个命题的条件和结论分别是另外一个命题的条件和结论的否认，则这两个命题称互为否命题。

We can form some new conditional statements starting with a conditional statement p->q. In particular, there are three related conditional statements that occur so often that they have special names. The proposition q->p is called the converse of p->q. The contrapositive of p->q is the proposition ¬q->¬p. The proposition ¬p->¬q is called the inverse of p->q.