"when does this calculation come up in physics, and with what slight modification?"
a' = [b x c]/[a*[(b x c)]], b' = [c x a]/[a*[(b x c)]], c' = [a x b]/a*[(b x c)]]
a* (b x c) does not equal 0 (* is dot product and (x) is cross product)
[b]2. Homework Equations
Show that...