5.20. Unit vectors $\bar{e}_{1}, \bar{e}_{2}, \bar{e}_{3}$ satisfy the condition $\bar{e}_{1}+\bar{e}_{2}+\bar{e}_{3}=\overline{0}$. Find $\bar{e}_{1} \bar{e}_{2}+\bar{e}_{2} \bar{e}_{3}+\bar{e}_{3} \bar{e}_{1}$.

## Group b
5.20. Indication. From the condition $\bar{e}_{1}+\bar{e}_{2}+\bar{e}_{3}=\overline{0}$ it follows that these vectors form an equilateral triangle and ( $=\left(\widehat{e_{3} ; \vec{e}_{1}}\right)=180^{\circ}-60^{\circ}=120^{\circ}$

Answer: $-1.5$.