title: Trigonometric identities
body: <p>I have an expression which has both sines and cosines and would like to write it using only sines (or cosines), possibly using the <a href="https://en.wikipedia.org/wiki/List_of_trigonometric_identities#Power-reduction_formula" rel="nofollow">power-reduction formula</a>.</p>
  
  <p>I tried to use SymPy but I cannot make it to "rewrite" to the desired output:</p>
  
  <pre><code>angle = symbols('angle')
  print (sin(angle)**2).rewrite(sin, cos) # (1 - cos(2*angle))/2
  print ((1 - cos(2*angle))/2).rewrite(cos, sin) # sin(angle)**2
  </code></pre>
  
  <p>Is there any way to tell Sympy to rewrite such expression using only sines (or cosines)?</p>