ef="http://en.wikipedia.org/wiki/Matrix\_multiplication">Wikipedia</a>
  for an explanation. See
  <a href="http://www.texample.net/tikz/examples/matrix-multiplication/">texample.net</a>
  for a visualization of the idea.</p>

  <div class="problem">

  <div class="marginpar">For extra practice, make up two small
    matrices and multiply them. Use <a
     href="https://sagecell.sagemath.org/?z=eJxztM1NLCnKrNCIjjbUMdYxiY3V5HJCiJnrGMXqKICkQJSukY4BSIGjlhMA16EPQw">Sage</a>
    or <a
     href="http://www.wolfr