input: Write a python function to calculate the area of a triangle using Heron's formula.
  assert area_triangle(3, 4, 5) == 6.0
  assert area_triangle(7, 10, 5) == 16.24807680927192
  assert area_triangle(6, 8, 10) == 24.0
output: import math
  
  def area_triangle(a, b, c):
      s = (a + b + c) / 2
      return math.sqrt(s * (s - a) * (s - b) * (s - c))