求积分∫∫|xy|dxdy=?,D为由圆x^2+y^2=a^2所围成的区域.∫∫的下面是D

∫∫|xy|dxdy=∫dθ∫│rcosθ*rsinθ│rdr (应用极坐标板换)=∫│cosθsinθ│dθ∫r³dr=(a^4/8)∫│sin(2θ)│dθ=(a^4/8)[∫sin(2θ)dθ-∫sin(2θ)dθ+∫sin(2θ)dθ-∫sin(2θ)dθ]=(a^4/16)(2+2+2+2)=a^4...