The radii of the ends of a frustum of a cone of slant height l are r₁ and r₂. The curved surface area of the frustum of the cone is

We know that if r₁ and r₂ are the radii of the two ends of the frustum and l is the slant height, then the curved surface area of a frustum is π(r₁ + r₂)l.

Where, $\mathrm{l}=\sqrt{{\mathrm{h}}^2+{\left({\mathrm{r}}_1-{\mathrm{r}}_2\right)}^2}$