EX =(a+b)/2 ->Er = [(1+3)/2]/2 DX = (b-a)^2/12 -> Dr = [(3-1)/2]^2 /12
ES=π[Er]^2=π[(1+3)/4]^2=π16/16=π
DS=π[Dr]^2=π[(2/2)^2]/12 = π/12
大概就是这个思路,计算我没去注意.