求阴影面积的图形一般都是看似不规则或规则而投有现成的公式去计算．只有通过仔细观察、分析,掌握一些常用的方法和技巧才有可能解决这类问题．1．和差法例1如图1,正方形ABCD的边长为a,以A为圆心,以线段AB为半径画弧BD,又分别以线段BC、CD为直径画半圆,则图中阴影部分的面积为____．解：依题设,图中阴影部分的面积为 Finding the shadow area of ​​the graph is generally seemingly irregular or regular and has a ready-made formula to calculate. Only by careful observation, analysis, master some common methods and techniques can solve this type of problem. 1. The difference law 1 is shown in Fig. 1. The side length of the square ABCD is a, the center of A is the arc, and the arc BD is the radius of the line segment AB. The semicircle is drawn with the diameters BC and CD, respectively. The area of ​​the shaded part is ____. Solution: According to the title, the area of ​​the shadow in the figure is