title: Polar equation of perimeter of half ellipse (concave mirror shape)
body: <p><code>var x = Cx + a * Math.cos(ang);</code></p>
  
  <p><code>var y = Cy + b * Math.sin(ang);</code></p>
  
  <p><code>Cx, Cy</code> are cords of center. <code>ang</code> is angle in <code>radians</code>. <code>a</code> is half of width, <code>b</code> is half of height.</p>
  
  <p>If I change values of <code>ang</code>, I get different points on circumference of ellipse. Below is the path, which I get with above equation.</p>
  
  <p><img src="https://i.stack.imgur.com/P3dgY.png" alt="Ellipse"></p>
  
  <p>But instead of this elliptical shape, I want something like half ellipse, something like concave mirror. Even if we stretch both of its end to infinity, they should not form elliptical shape.</p>
  
  <p><img src="https://i.stack.imgur.com/23SLm.png" alt="Concave"></p>
  
  <p>Can somebody provide me polar equations for second curve. I am very bad at digital drawing, but you can imagine that as concave mirror.</p>