title: Matlab 3D Plot with constraint
body: <p>i want to plot something like this:</p>
  
  <p>3*x*y</p>
  
  <p>with a constraint</p>
  
  <p>x^2+y^2=8</p>
  
  <p>in Matlab, so it should look like this:</p>
  
  <p><a href="http://www.wolframalpha.com/input/?i=maximize+3xy+on+x%C2%B2%2By%C2%B2%3D8" rel="nofollow noreferrer">http://www.wolframalpha.com/input/?i=maximize+3xy+on+x%C2%B2%2By%C2%B2%3D8</a></p>
  
  <p>any ideas?</p>
  
  <p>i tryed</p>
  
  <pre><code>ezsurf(@(x,y)3*x.*y)
  hold
  fimplicit(@(x,y)x.^2+y.^2-8)
  </code></pre>
  
  <p>but the result is bad, the line plot doesn't follow the surface</p>