You know that the lateral area of a regular pyramid, is [math]\frac{1}{2}p\ell[/math], where [math]p[/math] is the perimeter of the base and [math]\ell[/math] is the slant height. This formula can be derived by adding up the area of each lateral face. (See section 11.3). How about the lateral area of a cone? Two methods of deriving the formula ([math]\pi r \ell[/math]) are presented here.