'Gives the direction cosine between 2 points
Parameters
p : Point3D
Returns
list
Examples
>>> from sympy import Point3D
>>> p1 = Point3D(1, 2, 3)
>>> p1.direction_cosine(Point3D(2, 3, 5))
[sqrt(6)/6, sqrt(6)/6, sqrt(6)/3]'

def direction_cosine(self, point):

    a = self.direction_ratio(point)
    b = sqrt(Add(*((i ** 2) for i in a)))
    return [((point.x - self.x) / b), ((point.y - self.y) / b), ((point.z - self.z) / b)]
