叉乘
    vector 1: x1, y1, x2, y2
    vector 2: x1, y1, x3, y3

def cross_product (x1, y1, x2, y2, x3, y3):
    """ 叉乘
    vector 1: x1, y1, x2, y2
    vector 2: x1, y1, x3, y3
    """
    return (x2 - x1) * (y3 - y1) - (x3 - x1) * (y2 - y1)