The curved surface area of a cone's frustum is 

If $r_1$and $r_2$are the radii of the frustum's two ends and h is the vertical height, then Then the curved surface area of a frustum is $\pi \left(r_1+r_2\right)l$where l is the slant height of the cone and $I=\sqrt{h^2+{\left(r_1-r_2\right)}^2}$