input: Write a python function to calculate the area of a triangle given its three sides using Heron's formula.
  assert area_triangle(3, 4, 5) == 6.0
  assert area_triangle(7, 24, 25) == 84.0
  assert area_triangle(10, 10, 10) == 43.30127018922193
output: import math
  
  def area_triangle(a, b, c):
      s = (a + b + c) / 2
      area = math.sqrt(s * (s - a) * (s - b) * (s - c))
      return area