title: Looking for derivative script
body: <p>I am desperately looking for a JavaScript that can calculate the first derivative of a function. The function always includes only one variable, x.</p>
  
  <blockquote>
  <pre><code>e.g. f(x) = x²
    f'(3) = 2x
  </code></pre>
  </blockquote>
  
  <p>Consequently, the script should deliver the result  6, since <code>2*3 = 6</code>.</p>
  
  <p>I hope you know what I mean.</p>