title: Implicit differentiation with Python 3?
body: <p>How can we derivate a implicit equation in Python 3?<br>
  Example <code>x^2+y^2=25</code> differentiation is: <code>dy/dx=-x/y</code>, when try this:</p>
  
  <pre><code>from sympy import *
  
  init_printing(use_unicode=True)
  
  x = symbols('x')
  y = Function('y')(x)
  
  eq = x**2+y**2-25
  sol = diff(eq, x)
  print(sol)
  </code></pre>
  
  <p>But it shows:</p>
  
  <pre><code>2*x + 2*y(x)*Derivative(y(x), x)
  </code></pre>
  
  <p>How can get <code>-x/y</code>?</p>