input: Write a python function to calculate the area of a triangle given its three sides.
  assert triangle_area(3, 4, 5) == 6.0
  assert triangle_area(5, 12, 13) == 30.0
  assert triangle_area(7, 24, 25) == 84.0
output: def triangle_area(a, b, c):
      s = (a + b + c) / 2
      return (s * (s - a) * (s - b) * (s - c)) ** 0.5