L20A
       1.Review: The Geometric Meaning of the Second Fundamental Form
       2. Proposition: Meusnier
       3. Example: Unit Sphere
 
L20B
       1. Definition: Normal Section 
       2. Derivation: Define the Normal Curvature by Normal Section
 
L20C
       1. Definition: Principal Curvatures and Principal Directions
       2. Definition: Line of Curvature
       3. Proposition: Olinde Rodrigues
       4. Derivation: Euler Formula