input: Write a function to compute the area of a circle with the given radius. If the radius is a positive number return the area as a float, else return None.
  
  def area_circle(radius):
      if radius > 0:
          return math.pi * (radius ** 2)
      else:
          return None
  
  assert area_circle(5) == 78.53981633974483
  assert area_circle(-5) == None
  assert area_circle(0) == None
  assert area_circle(10) == 314.1592653589793
output: def area_circle(radius):
      if radius > 0:
          return math.pi * (radius ** 2)
      else:
          return None