∫∫{D}(XY ^2) dXdY= =∫{P→-P}Y ^2[∫{P/2→Y^2/(2P) }X dX]dY= =1/2∫{P→-P}[Y^4/(4P^2)-P^2/4]dY= =∫{0→-P}[Y^4/(4P^2)-P^2/4]dY= =(-P)^3/20-(-P)^3/4=-(-P)^3/5=P^3/5.