当x>=1时，y=4-x^2;当-1<=x<=1时，y=x^2-2当x<=-1时，y=4-x^2体积=π×3^2×4-∫（-2.-1）π（4-x^2)^2dx-∫（-1,1）π（x^2+2)^2dx-∫（1.2）π（4-x^2)^2dx=36π-(3又8/15)π-(11又1/15)π-(3又8/15)π=(17又13/15)π
