1.8. Find the cosine of the angle between the vectors with coordinates $\left(a_{1}, b_{1}, c_{1}\right)$ and $\left(a_{2}, b_{2}, c_{2}\right)$.
1.8. Since $( \mathbf{u}, \mathbf{v})=|\mathbf{u}| \cdot|\mathbf{v}| \cos \varphi$, where $\varphi-$ is the angle between vectors $\mathbf{u}$ and $\mathbf{v}$, the sought cosine of the angle is

$$
\frac{a_{1} a_{2}+b_{1} b_{2}+c_{1} c_{2}}{\sqrt{a_{1}^{2}+b_{1}^{2}+c_{1}^{2}} \sqrt{a_{2}^{2}+b_{2}^{2}+c_{2}^{2}}} .
$$