# 4.II.12A

Given a parametrized smooth embedded surface $\sigma: V \rightarrow U \subset \mathbf{R}^{3}$, where $V$ is an open subset of $\mathbf{R}^{2}$ with coordinates $(u, v)$, and a point $P \in U$, define what is meant by the tangent space at $P$, the unit normal $\mathbf{N}$ at $P$, and the first fundamental form

$E d u^{2}+2 F d u d v+G d v^{2} .$

[You need not show that your definitions are independent of the parametrization.]

The second fundamental form is defined to be

$L d u^{2}+2 M d u d v+N d v^{2},$

where $L=\sigma_{u u} \cdot \mathbf{N}, M=\sigma_{u v} \cdot \mathbf{N}$ and $N=\sigma_{v v} \cdot \mathbf{N}$. Prove that the partial derivatives of $\mathbf{N}$ (considered as a vector-valued function of $u, v$ ) are of the form $\mathbf{N}_{u}=a \sigma_{u}+b \sigma_{v}$, $\mathbf{N}_{v}=c \sigma_{u}+d \sigma_{v}$, where

$-\left(\begin{array}{cc} L & M \\ M & N \end{array}\right)=\left(\begin{array}{ll} a & b \\ c & d \end{array}\right)\left(\begin{array}{ll} E & F \\ F & G \end{array}\right)$

Explain briefly the significance of the determinant $a d-b c$.