Question

If A, B, C and D are any four points, then AB.CD + BC.AD + CA.BD is equal to 

(a) 1 

(b) 0 

(c) –1 

(d) None

Answer 1

Answer is (b) 0

Explanation:

Let D be the origin

and A = i, B = i + j, C = i + j + k

Now, AB.CD + BC.AD + CA.BD

= –1 + 0 + 1

= 0.