2.4 Exercise 26

Show that when $\overset{\rightarrow}{A} + \overset{\rightarrow}{B} = \overset{\rightarrow}{C}$, then $C^{2} = A^{2} + B^{2} + 2 A B \text{cos} \varphi$, where $\varphi$ is the angle between vectors $\overset{\rightarrow}{A}$ and $\overset{\rightarrow}{B}$.