def _signed_area_tri(points):
    """
    points : the coordinates of the triangle vertices -- (3x2) float array
    returns signed area of the triangle.

    """

    x1, y1 = points[0]
    x2, y2 = points[1]
    x3, y3 = points[2]

    return(((x1-x3)*(y2-y3)-(x2-x3)*(y1-y3))/2)