title: WolframAlpha: Solve Multiple Functions
body: <p>I'm trying to use WolframAlpha to solve for a variable.</p>
  
  <p>I have</p>
  
  <pre><code>u(k, r) = (900-3k)r^(k-1)
  </code></pre>
  
  <p>and</p>
  
  <pre><code>s(n, r) = sum u(k, r), k=1 to n
  </code></pre>
  
  <p>and I want to solve for r with</p>
  
  <pre><code>s(5000, r) = -600000000000
  </code></pre>
  
  <p>I've tried various incantations, but can't seem to get it working. I can't even get <code>s</code> defined to evaluate it.</p>
  
  <p>If you care, it is to solve this problem : <a href="http://projecteuler.net/index.php?section=problems&amp;id=235" rel="nofollow noreferrer">http://projecteuler.net/index.php?section=problems&amp;id=235</a></p>