Question

Let the angle between two nonzero vectors vector A and vector B be 120° and its resultant be vector C.

Answer 1

Here A,B,C are vectors 

|A+B|^2 = |A|^2 + |B|^2+ 2A.B            (1) 

|A-B|^2 = |A|^2 + |B|^2 - 2A.B             (2)

on subtracting (2) by (1)

|A+B|^2 - |A-B|^2 = 4A.B

so C^2 = |A-B|^2 + 4A.B

or C^2 = |A-B|^2 +4|A||B|cos120

since cos120 is negative so C will be less than |A-B|