Question

A x B = B x A, then the angle between A and B is

• A. $\pi$
• B. $\pi /3$
• C. $\pi /2$
• D. $\pi /4$

Answer 1

Answer: (A)

(A x B)=(B x A)
$\Rightarrow (A\times B)-(B\times A)=0[\because (B\times A)=-(A\times B)]$
(A x B) + (A x B) = 0
$\Rightarrow 2(A\times B)=0\Rightarrow 2ABsin\theta =0$
$sin\theta =0$
$\because [|A|=A\ne 0;|B|=B\ne 0]$
$\therefore \theta =0or\pi$