input: Write a python function to calculate the area of a triangle given three points.
  assert triangle_area((1, 4), (5, 4), (3, 2)) == 4.0
  assert triangle_area((0, 0), (0, 5), (5, 0)) == 12.5
  assert triangle_area((0, 0), (1, 1), (2, 0)) == 1.0
output: def triangle_area(p1, p2, p3):
      x1, y1 = p1
      x2, y2 = p2
      x3, y3 = p3
      return abs(x1*(y2 - y3) + x2*(y3 - y1) + x3*(y1 - y2)) / 2