∫∫(D)xdxdy=∫dθ∫r^2cosθdr (作极坐标变换)
=(8/3)∫(cosθ)^4dθ
=∫[1+(4/3)cos(2θ)+(1/3)cos(4θ)]dθ (应用倍角公式)
=π/2-π/4-2/3
=π/4-2/3.