input: Write a python function to calculate the area of a triangle given its three sides.
  assert area_of_triangle(3, 4, 5) == 6.0
  assert area_of_triangle(7, 8, 9) == 26.832815729997478
  assert area_of_triangle(10, 10, 10) == 43.30127018922193
output: def area_of_triangle(a, b, c):
      s = (a + b + c) / 2
      return (s * (s - a) * (s - b) * (s - c)) ** 0.5