平面几何证题中一个很难的地方,就是添作辅助线(直线、圆)。其中以辅助圆的添作更要求有一定的技巧,现在就其规律的探索举几例。一、看结论,想定理,根据已知,发现点共圆的条件。例1. Rt△ABC中,CD为斜边上的高,G为CD上的一点,AG的延长线和△ABC的外接圆交于H,则AG·AH=AD·AB(统编教材《第五册》) One of the most difficult places in the plane geometry test is to add auxiliary lines (lines, circles). Among them, the addition of an auxiliary circle requires more skill, and now it gives a few examples of its regular exploration. First, look at the conclusion, think of the theorem, according to the known, discovered the conditions of the point of co-circulation. Example 1. In RtΔABC, CD is the height on the hypotenuse, G is the point on the CD, the extension line of AG and the circumcircle of △ABC are intersected at H, then AG·AH=AD·AB (the textbook) Volume 5)