title: get polynom representation for lagrangian interpolation
body: <p>How can I represent an incomplete mathematical function?</p>
  
  <p>I need to do something like <code>(x - constant)</code> then </p>
  
  <pre><code>(x - constant)*(x - another) =&gt; (x^2 - x * constant - x * another + constant * another)
  </code></pre>
  
  <p>and so on.</p>
  
  <p>I'm trying to make a program to do Lagrangian interpolation (finding a function for some points)
  so I need to make a function that I can see (print, or something), from a set of known values.
  sorry if confusing.</p>