L8A
      1. Two Subclasses of Developable surface
 
L8B
      1. Two Subclasses of Developable surface (cont.)
      2. Example: The Envelope of the Family of Tangent Planes
          Along a Curve of  a Surface
 
L8C
      1. The Envelope of the Family of Tangent Planes Along a
          Curve of a Surface Is Developable