解某些三角问题时 ,常规的思考方法是由条件到结论的定向思考 .但有些问题按照这样的思维方式来寻求解题的途径比较困难 ,甚至无从下手 .在这种情况下 ,经常要求我们改变思维方向 ,换一个角度思考 ,以找到一条绕过障碍的新的途径 .1　构造函数例 1 　 (1 994年全国高中数学联赛 When solving certain trigonometric problems, the conventional thinking method is the directional thinking from the condition to the conclusion. However, some problems in this way of thinking to find the solution to the problem are more difficult, even impossible to start. In this case, often ask us Change the direction of thinking and think from another perspective to find a new way to get around obstacles. 1 Constructor Example 1 (1 994 National High School Mathematics League