input: Write a python function to calculate the area of a triangle given the three side lengths.
  assert area_triangle(5, 9, 11) == 21.19895788082824
  assert area_triangle(3, 4, 5) == 6.0
  assert area_triangle(7, 10, 5) == 16.24807680927192
output: def area_triangle(a, b, c):
      s = (a + b + c) / 2
      return (s * (s - a) * (s - b) * (s - c)) ** 0.5