计算阴影阴部分的面积。'@)B/T5B;U2z+K8F.W+_
'@)B/T5B;U2z+K8F.W+_
算式：提示-e,q*G:\(r2W8}7J3q'@)B/T5B;U2z+K8F.W+_
=4b/a4D1N9x(F)M0T0E:e4K7W（cm2）'@)B/T5B;U2z+K8F.W+_