几何直线型问题中常需推证角相等,有些题目要添加辅助线才能得证,解题时若题目中具有四点共圆的条件特征,可以利用四点共圆这个隐藏性的条件推证出两角相等,对于解决直线型问题是一个很好的工具.例1(2016朝阳区八下期末)在等腰直角三角形ABC中,∠ACB=90°,AC=BC,直线l过点C且与AB平行.点D在直线l上(不与点C重合),作射线DA.将射线DA绕 Geometrical linear problems often need to deduce that the angles are equal. Some questions need to be supplemented by auxiliary lines in order to obtain the proof. If there are four points in a circle, the conditional characteristics of the problem can be solved by using the hidden condition The same two corners are good tools to solve the linear problem. Example 1 In the isosceles right triangle ABC, ∠ACB = 90 °, AC = BC, the straight line l crosses point C and And AB parallel point D in the straight line l (not coincident with the point C), for the ray DA. The ray DA around