∫∫D根号(x^2+y^2)dσ=2∫(0,π/2)dθ∫(0,2cosθ)r^2dr=2∫(0,π/2)(1/3)(2cosθ)^3dθ=(16/3)∫(0,π/2)(cosθ)^3dθ=(16/3)(2/3)=32/9
