平面几何中常出现“三角形的一个内角为另一个内角的二倍”即“二倍角”条件的问题,学生遇到这样的问题,感觉难度很大,无从下手.其实,这类问题添加辅助线是有一定技巧和规律的,可归纳总结为以下四种方法:1.做大角的平分线;2.以小角的顶点、小角的一边为一边,在小角的外部做一个 Plane geometry often appears in the “triangle one corner of the other two times the angle of” double angle "condition, the students encounter such a problem, I feel very difficult to start .In fact, such problems add the auxiliary line is There are certain skills and laws, can be summarized in the following four ways: 1. Big angle bisector; 2. Apex of the small angle, one side of the small angle for one side, outside the small angle to do a