input: Write a python function to compute the area of a circle given its radius.
  assert area_circle(1) == math.pi * 1 ** 2
  assert area_circle(3) == math.pi * 3 ** 2
  assert area_circle(5) == math.pi * 5 ** 2
output: import math
  
  def area_circle(radius):
      return math.pi * radius ** 2