"review-matrices">Matrices</h1>
  <p>We will soon discover
  that matrices represent derivatives in high dimensions. When you
  use matrices to represent derivatives, the chain rule is
  precisely matrix multiplication. For now, we just need to become
  comfortable with matrix multiplication. We perform matrix
  multiplication &ldquo;row by column&rdquo;. Wikipedia has an excellent
  visual illustration of how to do this. See
  <span class="marginpar">Click on the links to go to the websites.</span>
  <a hr