我们知道,命题是由条件和结论两个部分构成,通常表述为：“如果A,那么B”的形式．其中A是命题的条件,B是命题的结论．譬如：两条直线被第三条直线所截,如果内错角相等,那么两条直线平行．其中“内错角相等”是条件,“两条直线平行”是结论．若将一个命题的条件和结论互相交换,那么所得的命题就是原命题的逆命题．即“如果A,那么B”的逆命题是“如果B,那么A”． We know that the proposition consists of two parts: condition and conclusion. It is usually expressed as: if A, then B. Where A is the condition of the proposition and B is the conclusion of the proposition. For example: two straight lines are truncated by a third straight line. If the internal angles are equal, then the two straight lines are parallel. The “equal inside angle is equal” is the condition, “two straight lines are parallel” is the conclusion. If the conditions and conclusions of a proposition are exchanged, the resulting proposition is the converse proposition of the original proposition. That is, if A, then B, the reverse proposition is “if B, then A”.