（定积分的应用）所求环体的体积=∫[π(5+√(16-x²))²-π(5-√(16-x²))²]dx=40π∫√(16-x²)dx=40π∫4cost*4costdt (令x=4sint)=320π∫[1+cos(2t)]dt (应用倍角公式)=320π[t+sin(2t)/2]...