input: Write a function to calculate the area of a triangle using Heron's formula.
  assert triangle_area(3, 4, 5) == 6.0
  assert triangle_area(7, 24, 25) == 84.0
  assert triangle_area(10, 10, 10) == 43.30127018922193
output: def triangle_area(a, b, c):
      s = (a + b + c) / 2
      return (s * (s - a) * (s - b) * (s - c)) ** 0.5