Chapter 2, Problem 77AP

Show that when $\stackrel{\to }{\text{A}}+\stackrel{\to }{\text{B}}=\stackrel{\to }{\text{C}}$ then $A^2+B^2+2AB\cos \phi$ , where $\phi$ is the angle between vectors $\stackrel{\to }{\text{A}}$ and $\stackrel{\to }{\text{B}}$ .