If the coordinates of the points A, B, C, D be (1, 2, 3), (4, 5, 7), (– 4, 3, – 6) and
(2, 9, 2) respectively, then find the angle between the lines AB and CD 6 If the lines 
1
2
3
1
1
6
and
3
2
2
3
1
5
x
y
z
x
y
z
k
k
−
−
−
−
−
−
=
=
=
=
−
−
 are perpendicular,,
find the value of k 7