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 Therefore,   cos θ = 
1
2
1
2
1
2
2
2
2
2
2
2
1
1
1
2
2
2
A A
B B
C C
A
B
C
A
B
C
+
+
+
+
+
+
�Note
1 If the planes are at right angles, t hen θ = 90o and so cos θ = 0