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## Homework Statement

Giving 2 closed curves in 3-dimension space C1 and C2, prove that:[tex]\oint _{C1} \oint _{C2}\frac{(\vec{dl_2}.\hat{r_{12}})\vec{dl_1}}{r^2_{12}}=0[/tex]

Where:

_ [tex]\vec{dl_1}[/tex] and [tex]\vec{dl_2}[/tex] are the vector elements of the curves C1 and C2 respectively.

_ [tex]r_{12}[/tex] is the distance between the two above elements.

_ [tex]\hat{r_{12}}[/tex] is the unit vector of vector [tex]\vec{r_{12}}[/tex]

## Homework Equations

No idea.

## The Attempt at a Solution

No idea too. It's out of my knowledge and ability. This is not a homework question, but on the way to solve a physics problem, I encountered this. Any idea about the way to solve would be very much appreciated. Thank you!