input: Write a function to calculate the area of a triangle given its three sides using Heron's formula.
  assert triangle_area(3, 4, 5) == 6.00
  assert triangle_area(7, 24, 25) == 84.00
  assert triangle_area(5, 12, 13) == 30.00
output: def triangle_area(a, b, c):
      s = (a + b + c) / 2
      area = (s * (s - a) * (s - b) * (s - c)) ** 0.5
      return round(area, 2)