input: Write a python function to calculate the area of a circle given its radius.
  assert area_circle(5) == 78.53981633974483
  assert area_circle(10) == 314.1592653589793
  assert area_circle(1) == 3.141592653589793
output: import math
  
  def area_circle(radius):
      return math.pi * (radius ** 2)