If $\vec a,\vec b,\vec c, \vec d$ are coplanar vectors, then $(\vec a×\vec b)×(\vec c×\vec d)=$

1 $\vec a$ $\vec b$ $\vec 0$

$\vec 0$

Since $\vec a,\vec b,\vec c, \vec d$ are coplanar vectors. Therefore, $\vec a×\vec b$ and $\vec c×\vec d$ are vectors normal to the plane containing the four vectors. $∴\vec a×\vec b$ is parallel to $\vec c×\vec d$ $⇒(\vec a×\vec b)×(\vec c×\vec d)=\vec 0$