input: Write a python function to calculate the area of a triangle given its vertex coordinates.
  assert area(4, 7, 5, 6, 7, 4) == 3.0
  assert area(0, 0, 1, 0, 0, 1) == 0.5
  assert area(1, 1, 2, 2, 3, 1) == 1.0
  assert area(0, 0, 0, 0, 0, 0) == 0.0
output: def area(x1, y1, x2, y2, x3, y3):
      return abs((x1*(y2 - y3) + x2*(y3 - y1) + x3*(y1 - y2)) / 2.0)