在有些几何问题中,常通过在原图形上添加辅助线,把其补成一个新的特殊的几何图形,如等边三角形、正方形、矩形、圆等.利用这些特殊的新图形的性质来求出原图形的有关结果的方法称为补形法,往往会使解法简捷明快,下面举例说明. 例一如图1,已知在四边形ABCD中,∠B=∠D=90°,∠A=135°,AD=2 3,BC In some geometric problems, it is often used to add auxiliary lines to the original graphics to make up a new special geometry, such as equilateral triangles, squares, rectangles, circles, etc. These are used to determine the nature of these special new graphics. The method of the related result of the original figure is called the complement method, which often makes the solution simple and straightforward. The following example is illustrated. Example 1 As shown in Figure 1, it is known that in the quadrilateral ABCD, ∠B=∠D=90°, ∠A=135 °, AD=2 3, BC