R= a2 k S dx/=8(R-)/≈6·2r2 dt k△ 16G 16G 3T 3=3T 4G ∫a(a+am2+ka-a2) 4G ∫a(a2+k- 3 ∠s3(m2+kan 4G 2021/2/5 Zhang De-Hai2021/2/5 Zhang De-Hai 5