input: Write a function to calculate the area of a circle given its radius. The function should return the area using the mathematical constant pi from the math module.
  
  assert area_circle(5) == 78.54
  assert area_circle(0) == 0.00
  assert area_circle(1) == 3.14
  assert area_circle(7.5) == 176.71
output: import math
  
  def area_circle(radius):
      return math.pi * radius ** 2