Say we have an orthonormal basis in RR^3 {A,B,C}. Then when taking the cross product of any 2 of these, we know it is equal to either the third basis element, or −1 times that element. Say we are given A xx B=C, and we want to know whether A xx C=B or A xx C=−B. Via the right-hand rule, I can see that it should be −B, but how can I show this algebraically, using only what is given here?

Audrey Arnold

Audrey Arnold

Answered question

2022-11-11

Say we have an orthonormal basis in R 3 {A,B,C}. Then when taking the cross product of any 2 of these, we know it is equal to either the third basis element, or −1 times that element.
Say we are given A × B = C, and we want to know whether A × C = B or A × C = B
Via the right-hand rule, I can see that it should be −B, but how can I show this algebraically, using only what is given here?

Answer & Explanation

Izabella Henson

Izabella Henson

Beginner2022-11-12Added 20 answers

You can use the vector triple product identity:
x × ( y × z ) = ( x z ) y ( x y ) z
Then
A × C = A × ( A × B ) = ( A B ) A ( A A ) B = 0 A 1 B = B

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