'The equation of the line in 3D
Parameters
x : str, optional
The name to use for the x-axis, default value is \'x\'.
y : str, optional
The name to use for the y-axis, default value is \'y\'.
z : str, optional
The name to use for the x-axis, default value is \'z\'.
Returns
equation : tuple
Examples
>>> from sympy import Point3D, Line3D
>>> p1, p2 = Point3D(1, 0, 0), Point3D(5, 3, 0)
>>> l1 = Line3D(p1, p2)
>>> l1.equation()
(x/4 - 1/4, y/3, zoo*z, k)'

def equation(self, x='x', y='y', z='z', k='k'):

    (x, y, z, k) = (_symbol(x), _symbol(y), _symbol(z), _symbol(k))
    (p1, p2) = self.points
    a = p1.direction_ratio(p2)
    return (((x - p1.x) / a[0]), ((y - p1.y) / a[1]), ((z - p1.z) / a[2]), k)
