e
    is perpendicular to it, 
    so the unit normal function is then found by 
    (<a href="#apostol2007calculus">Apostol, 2007</a>):

    $$\mathbf{n}(t) = \frac{\mathbf{e}'(t)}{\left\Vert\mathbf{e}'(t)\right\Vert}$$
  </p>
    <p>
    We can also observe that 
    the unit normal is just a 90 degree rotation of the unit tangent,
    so we can obtain the unit normal function simply as:

    $$\begin{array}{l}
    n_x(t) = e_y(t) \\
    n_y(t) = -e_x(t) \\
    \end{array}$$
  </p>
    <h2>
      <a name=