input: Write a function to compute the area of a circle given its radius. Use the formula area = π * radius^2. If the radius is not provided, return the area of a circle with a radius of 1.
  assert area_circle(5) == 78.54
  assert area_circle(3) == 28.27
  assert area_circle() == 3.14
output: def area_circle(radius=1):
      return 3.14 * radius ** 2