qqp question1: If 2 sides of a triangle are described by 2 vectors a and b, how does one work out what vector defines the third side? question2: If a, b and c represent the vectors determined by the sides of a triangle ABC (taken in order), then how I prove that |b×c|=|c×a|=|a×b| and hence? How do I show that (a-b) ×(a+b) =2(a×b)?