divergence of the cross product of two vectors proof

Question

Prove $vec{nabla} cdotleft ( vec{A}timesvec{B} right )=vec{B}cdotleft ( nabla timesvec{A} right )-vec{A}cdotleft ( nabla timesvec{B} right )$

I have expanded the LHS for this and obtain a horrible expression that would be even tedious to put them into latex form without a headache.

I attempted to rearrange the LHS so as to obtain the RHS but to no avail.

any help is appreciated.

KittyL · Accepted Answer

You just need to write everything out neatly, so you can see the equating terms.

Let's deal with the left hand side first.
$$vec{A}timesvec{B}=(A_2B_3-A_3B_2)vec{i}+(A_3B_1-A_1B_3)vec{j}+(A_1B_2-A_2B_1)vec{k}$$
I wrote the component of $A$ always in front of the component of $B$ in order to see easily.
Now the whole left hand side is the divergence of this:
$$frac{partial(A_2B_3-A_3B_2)}{partial x}+frac{partial(A_3B_1-A_1B_3)}{partial y}+frac{partial(A_1B_2-A_2B_1)}{partial z}$$

Let's wait for a while to do the product rule, and instead, look at the right hand side. 
$$nabla times vec{A}=(frac{partial A_3}{partial y}-frac{partial A_2}{partial z})vec{i}+(frac{partial A_1}{partial z}-frac{partial A_3}{partial x})vec{j}+(frac{partial A_2}{partial x}-frac{partial A_1}{partial y})vec{k}$$
The first term is the dot product of $B$ with it, so just replace the $i,j,k$ by $B_1,B_2,B_3$:
$$vec{B}cdot nabla times vec{A}=B_1(frac{partial A_3}{partial y}-frac{partial A_2}{partial z})+B_2(frac{partial A_1}{partial z}-frac{partial A_3}{partial x})+B_3(frac{partial A_2}{partial x}-frac{partial A_1}{partial y}) $$

Now the second term is similar. We just need to switch $A$ and $B$, and remember it is negative:
$$vec{A}cdot nabla times vec{B}=A_1(frac{partial B_3}{partial y}-frac{partial B_2}{partial z})+A_2(frac{partial  B_1}{partial z}-frac{partial B_3}{partial x})+A_3(frac{partial B_2}{partial x}-frac{partial B_1}{partial y}) $$

Now you can compare each product in the left hand side with the corresponding terms in the right hand side. For example, the first product rule in the left hand side is
$$frac{partial(A_2B_3)}{partial x}=A_2frac{partial(B_3)}{partial x}+frac{partial(A_2)}{partial x}B_3$$

It is easy to see which terms in the right hand side equal to these two terms. I'll leave it to you to proceed.

Herman Jaramillo · Answer

In index notation

begin{eqnarray*}
  (A times B)_{i} = epsilon_{ijk} A_j B_k
end{eqnarray*}
(Einstein's convention of sum over repeated indices).
Then if $A_{j_{,i}}=partial A_j/partial x_i$,
and from $nabla times A=epsilon_{ijk} A_{k_{,j}}$ (and so for the other symbols)

begin{eqnarray*}
  nabla cdot (A times B) &=&   [epsilon_{ijk} A_j B_k],_{i} \
  &=& epsilon_{ijk} A_{j_{,i}} B_k + epsilon_{ijk} A_j B_{k_{,i}}  \
  &=& B_k ( epsilon_{kij} A_{j_{,i}})  - A_j ( epsilon_{jik} B_{k_{,i}})  \
  &=& B cdot (nabla times A) - A cdot  ( nabla times B )
end{eqnarray*}
where the minus ``-'' sign appears since $epsilon_{ijk}=-epsilon_{jik}$.

divergence of the cross product of two vectors proof

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