title: finding crital value in python, problem with finite set
body: <p>I've written program for finding critical value in Python.</p>
  
  <p>My code:</p>
  
  <pre class="lang-py prettyprint-override"><code>from sympy import *
  def find_critical_points(f, x):
      fd = diff(f)
      dRoots = solveset(fd, x)
      a = Rational(float(dRoots))
      return a
  </code></pre>
  
  <p>And I wrote test for that:</p>
  
  <pre class="lang-py prettyprint-override"><code>x = Symbol('x')
  lst = find_critical_points(x**4+x**3, x)
  assert lst == [-3/4,0]
  lst = find_critical_points(x,x)
  assert lst == []
  </code></pre>
  
  <p>Python return me error:</p>
  
  <blockquote>
    <p>float() argument must be a string or a number, not 'FiniteSet'</p>
  </blockquote>
  
  <p>Please help with this error.</p>