If the vectors `overset(to)(b), overset(to)(c ) , overset(to)(d)` are not coplanar then prove than the vectors `(overset(to)(a) xx overset(to)(b)) xx (overset(to)(c ) xx overset(to)(d)) + (overset(to)(a) xx overset(to)(c )) xx (overset(to)(d) xx overset(to)(b))`
`+(overset(to)(a) xx overset(to)(d)) xx (overset(to)(b) xx overset(to)( c))` is parallel to `overset(to)(a) `