4. The triple scalar product is defined by It is known that if the triple scalar product of 3 vectors is equal to zero then the3 vectors are coplanar (i.e.

all 3 vectors are on the same plane). \text { a) Show that the } 3 \text { vectors } \vec{u}=[1,2,-3], \vec{v}=[0,1,1] \text { and } \vec{w}=[2,1,-9] \text { are coplanar. } b) Using geometrical reasoning, explain why if ū, v, and w are coplanar, then ū × v · w = 0.