1
$\begingroup$

If I have vectors $a, b, c \in \mathbb{R}^3$, and if we have e.g. $a = b\times c$, is there any way to express $b$ in terms of the other two?

$\endgroup$
  • 1
    $\begingroup$ Answer : NO, because for a fixed pair $(a,c)$ you have several $b$ satisfying the equation. For example, if $a=0$, any $b$ collinear to $c$ will do. $\endgroup$ – Ewan Delanoy May 13 '13 at 11:12
  • $\begingroup$ okay I should exclude linear dependence between all of them . Does this change something? $\endgroup$ – user66906 May 13 '13 at 11:15
1
$\begingroup$

Write out the vectors as $a=(a_1,a_2,a_3)$, and so on, do the cross product, set the two sides equal, and you'll get a system of three linear equations in the three unknown components of $b$.

$\endgroup$
1
$\begingroup$

No, it's not possible, since infinitely many values of $b$ lead to the same value of $a$. This is obvious for $c=0$. For $c\neq0$ both $b_1=b$ and $b_2=b+c$ give the same $a$: $$ b_2\times c = (b+c)\times c = b\times c + c\times c = b\times c = b_1\times c. $$

$\endgroup$
1
$\begingroup$

To elaborate on @Ralph Tandetzky's answer: Interpreting the cross product geometrically, assuming $a\ne 0$, $b$ must be in the plane orthogonal to $a$, and the signed area of the parallelogram spanned by $b$ and $c$ must be $\|a\|$. Thus, there is a line — parallel to $c$ — of solutions.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy