
Show that if u,v, and w are vectors in 3D with,

The vectors u,v and w form the directed sides of a triangle.
Thus, the equality of the cross products of two of its sides
(disregarding orientation) follows from the geometric definition
of the cross product. The RHS rule shows that this are the products
with vectors comming out of the plane.

Take crossprod with u on both sides of the first equation to obtain
