当我们直接从正面考虑不易解决问题时,就要改变思维方向,从结论入手,反面思考.这种从“正面难解决就从反面思考”的思维方式就是我们通常所说的间接解法中的一种——反证法.反证法是肯定题设而否定结论,从而导出矛盾的推理方法.用反证法完成一个命题的证明,一般有以下三步步骤: When we think directly from the front is not easy to solve the problem, we must change the direction of thinking, starting from the conclusion, the opposite thinking .This from the “positive solution is difficult to think from the opposite” way of thinking is what we usually call indirect solution Is a kind of negative proof.Reverse proof is affirmative and negative conclusion, thus deriving contradictory reasoning method.With the proof of proof to complete a proposition, there are generally three steps: