title: Sympy, How to get the solution of an ODE in a certain form
body: <p>Let's say we have this ODE :</p>
  
  <p><img src="https://chart.googleapis.com/chart?cht=tx&amp;chl=9.6f%28t%29%20%2B%208.0%5Cfrac%7Bd%5E2%7D%7Bdt%5E2%7Df%28t%29%3D0" alt="19.6f(t) + 8.0\frac{d^2}{dt^2}f(t)=0"></p>
  
  <p>The solution of this equation normally is </p>
  
  <p><img src="https://chart.googleapis.com/chart?cht=tx&amp;chl=f%28t%29%3DC_1sin%287%5Csqrt%7B5%7Dt%2F10%29%2BC_2cos%287%5Csqrt%7B5%7Dt%2F10%29" alt="f(t)=C_1sin(7\sqrt{5}t/10)+C_2cos(7\sqrt{5}t/10)"></p>
  
  <p>Which is what sympy will give me obviously, though I need the solution in this form instead :</p>
  
  <p><img src="https://chart.googleapis.com/chart?cht=tx&amp;chl=f%28t%29%3DAcos%287%5Csqrt%7B5%7Dt%2F10%20%2B%20%5Cphi%29" alt="f(t)=Acos(7\sqrt{5}t/10 + \phi)"></p>
  
  <p>With the constants being A and <img src="https://chart.googleapis.com/chart?cht=tx&amp;chl=%5Cphi" alt="\phi">.</p>
  
  <p>The goal of this is to study the dephasing of the system in question.</p>