18
© NCERT
not to be republished
THREE D IMENSIONAL G EOMETRY
489
If 
1nr  and 
2nr  are normals to the planes and θ be the angle between the planes
r n⋅1
r r  = d1 and 
2
2 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