解题活动的实质是思维的转化过程,由条件A推出B,记为AB,则A是B的充分条件;由B推出A,记为BA,则A是B的必要条件.在转化的过程中如果能保证推出的等价性,即AB,那当然最为理想.如果寻求原问题的充要条件比较困难,或所寻求的充要条件很繁,不便于求解,可以退而求其次,先寻找结 The essence of the problem-solving activity is the transformation process of thinking, which is introduced by condition A, B is written as AB, then A is a sufficient condition of B; B is introduced by A, denoted by BA, then A is a necessary condition of B. In the process of conversion if we can guarantee the introduction of equivalence, that is, A  B, of course, the most ideal.If seeking the original problem of the necessary and sufficient conditions, or seek the necessary and sufficient conditions are not easy to solve, you can Back to seek second, first find the knot