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 Hence, cos θ = A1A2 + B1B2 + C1C2 = 0 2