input: Write a python function to calculate the area of a triangle using Heron's formula.
  assert heron(6, 8, 10) == 24.0
  assert heron(3, 4, 5) == 6.0
  assert heron(10, 10, 10) == 43.30127018922193
output: def heron(a, b, c):
      s = (a + b + c) / 2
      area = (s * (s - a) * (s - b) * (s - c)) ** 0.5
      return area