1. The problem statement, all variables and given/known data If a, b, c, d are all vectors contained in the same plane, explain why (a X b) X (c X d) = <0,0,0> 2. Relevant equations The Cross Product! 3. The attempt at a solution I know that since all of the vectors are in the same plane that means that a cross product between any of the vector will either be parallel, or anti-parallel. In this case the two cross products done create a vector with an angle of 180 degrees between them. So when they are crossed the result is 0. I just do not know how to "mathematically" prove this.