title: Solve an equation using a python numerical solver in numpy
body: <p>I have an equation, as follows:</p>
  
  <p><code>R - ((1.0 - np.exp(-tau))/(1.0 - np.exp(-a*tau))) = 0</code>.</p>
  
  <p>I want to solve for <code>tau</code> in this equation using a numerical solver available within numpy. What is the best way to go about this? </p>
  
  <p>The values for <code>R</code> and <code>a</code> in this equation vary for different implementations of this formula, but are fixed at particular values when it is to be solved for tau. </p>