内环直径=0.4-0.1-0.1=0.2
所以内环半径=0.22=0.1
外环半径=0.42=0.2
所以面积=大圆面积-小圆面积
=0.2*0.2*3.14-0.1*0.1*3.14
=(0.04-0.01)*3.14
=0.0942