Let ABC be a cone. A frustum DECB is cut by a plane parallel to its base. Let r1 and r2 be the radii of the ends of the frustum of the cone and h be the height of the frustum of the cone.
In ΔABG and ΔADF, DF||BG
∴ ΔABG ∼ ΔADF
CSA of frustum DECB = CSA of cone ABC − CSA cone ADE
CSA of frustum = π(r1+r2)l
Total surface area of frustum
=CSA of frustum + Area of upper circular end + Area of lower circular end
