联立解 y = x² 与 y = 4-x² 得交点 (-√2, 2), (√2, 2) Vx = π∫<-√2, √2>[(4-x^2)^2 - x^4]dx = 2π∫<0, √2>(16-8x^2)dx= 16π[2x-x^3/3)]<0, √2> = (64√2/3)π
