Arun
Last Activity: 7 Years ago
Dear Akshat
we have
(a x b) x (r x c)
= ((a x b ) . c) r – ((a x b) . r) c
= [a b c] r – [a b r] c
similarly
(b x c) x (r x a) = [b c a] r – [b c r] c
and
(c x a) x (r x b) = [c a b] r – [c a r] b
when we add all three
we get
3 [a b c] r – [a b c] r = 2 [a b c] r
hence option 3 is correct
reagrds
Arun (askIITians forum expert)