The angle between vectors a and b is 60 and |a| = 2, |b| = 1. Show that |2a – b| = sqrt (13) and |2a + b| = sqrt (21). Do we have to use dot product? How do we find the magnitude of vector (2a + b)? I received this question from a student from Nanyang Junior College and would like to share this approach with all of you. I hope the diagram explains it clearly.