4r/3 :设内接于球的圆锥高为h,则圆锥底半径为p,有p^2=r^2-(h-r)^2=2rh-h^2,体积为V=3.14*(2rh-h^2)*h/3,取其导数,当V`=0,即h=4r/3时,体积取极值,(也是最大值);此时V=3.14*[2r*4r/3-(4r/3)^2]*(4r/3)/3=3.14*8*r^3/27=2*V球/9