1. The problem statement, all variables and given/known data Let s and t be scalars. show that u, v and su + tv are coplanar. u and v are arbitrary vectors, 2. Relevant equations 3. The attempt at a solution Well, i suppose if they are coplanar, triple scalar product of them should be 0, right? and su+tv is evidently a linear combination of u and v. but i got a little confused when vectors are arbitrary and not really sure how to show it properly. any help would be appreciated, thanks.