d
n
r
r=
r Then θ is the angle between the normals to the planes drawn from some common
point We have,
cos θ =
1
2
1
2
|
| |
|
n
n
n
n
r⋅
r
r
r
�Note  The planes are perpendicular to each other if 
1nr 2nr  = 0 and parallel if
1nr  is parallel to  
2nr