input: Write a python function to calculate the area of a circle given its radius.
  assert area_of_circle(3) == 28.274333882308138
  assert area_of_circle(5) == 78.53981633974483
  assert area_of_circle(10) == 314.1592653589793
output: def area_of_circle(radius):
      import math
      return math.pi * radius * radius