input: Write a python function to calculate the area of a triangle given its three vertices.
  assert triangle_area((2, 0), (-2, 0), (0, 4)) == 8.0
  assert triangle_area((1, 1), (3, 5), (6, 2)) == 7.5
  assert triangle_area((0, 0), (0, 5), (5, 0)) == 12.5
output: def triangle_area(vertex1, vertex2, vertex3):
      x1, y1 = vertex1
      x2, y2 = vertex2
      x3, y3 = vertex3
      area = abs(x1*(y2 - y3) + x2*(y3 - y1) + x3*(y1 - y2)) / 2.0
      return area