input: Write a function to prompt the user to enter the value of radius to calculate the area of a circle using the following formula area = Pi * radius^2. Return the calculated area.
  assert calculate_circle_area(5.0)==78.5
  assert calculate_circle_area(7.5)==176.625
  assert calculate_circle_area(3.2)==32.1536
  assert calculate_circle_area(0)==0.0
  assert calculate_circle_area(-3.4) is None
output: def calculate_circle_area(radius):
      if radius < 0:
          return None
      return 3.14 * (radius ** 2)