title: calculate an integral in python
body: <p>I need to calculate an integral in python. </p>
  
  <p>I have imported sympy.</p>
  
  <pre><code> g(a,z) = integral_from_z_to_inf of ( t^(a-1) * e^(-1))
  </code></pre>
  
  <p>in python: </p>
  
  <pre><code> x,a,z = symbols('x a z')
   g = integrate(x**(a-1) * exp(-x), z, oo)
  </code></pre>
  
  <p>I got error: </p>
  
  <pre><code> ValueError: Invalid limits given: (z, oo)
  </code></pre>
  
  <hr>
  
  <p>I called: </p>
  
  <pre><code>b,c,mean,variance  = S('b c mean variance'.split())
  
  ff1 = b*g((1+c), lb / b)  // lb is a constant, b and c are unknown var that I need to solve. and mean and variance are constants. 
  </code></pre>
  
  <p>I got error: </p>
  
  <pre><code>TypeError: 'Add' object is not callable
  </code></pre>