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 Cartesian form Let θ be the angle between the planes,
A1 x + B1 y + C1z + D1 = 0 and  A2x + B2 y + C2 z + D2 = 0
The direction ratios of the normal to the planes are A1, B1, C1 and A2, B2, C2
respectively