Let the angle between two non-zero vectors A and B be 120° and its resultant be C.

It is given thatA·B=|A|Bcos120°=-12A|B|C=A+BNow it can be written as: C2=(A+B)·(A+B)=A2+B2+2A·B =A2+B2-|A‖B|………….this is equation 1|A-B|2-C2= equation (2-1)=2|A‖B|∴C2