cos φ =

$$cos \varphi = \frac{l_1l_2 + m_1m_2}{\sqrt{l_1^2 + m_1^2} \cdot \sqrt{l_2^2 + m_2^2}}$$

Ответ: $$\frac{l_1l_2 + m_1m_2}{\sqrt{l_1^2 + m_1^2} \cdot \sqrt{l_2^2 + m_2^2}}$$