title: Area of Intersection between Two Circles
body: <p>Given two circles:</p>
  
  <ul>
  <li>C1 at (<code>x1</code>, <code>y1</code>) with <code>radius1</code></li>
  <li>C2 at (<code>x2</code>, <code>y2</code>) with <code>radius2</code></li>
  </ul>
  
  <p>How do you calculate the area of their intersection? All standard math functions (<code>sin</code>, <code>cos</code>, etc.) are available, of course.</p>