The height of the cone is sqrt(20^2-8^2), because the slant length is represented by the hypotenuse of a right-angled triangle and the radius of the base is another side. The height is the third side=sqrt(400-64)=sqrt(336)=4sqrt(21). The area of the base is (pi)8^2=64(pi). The vol of the cone is 256(pi)sqrt(21) cc.