input: Write a function to find the volume of a cone.
  assert volume_cone(5,12)==314.15926535897927
  assert volume_cone(10,15)==1570.7963267948965
  assert volume_cone(19,17)==6426.651371693521
  
output: def volume_cone(radius, height):
      return (1/3) * 3.141592653589793 * radius**2 * height