3-39 할리데이 10판 솔루션 일반물리학

$$\begin{cases} \vec A:(6.00,\theta_A)\\ \vec B:(7.00,\theta_B)\\ \vec A\cdot\vec B=14.0\\ \end{cases}$$ $$\phi_{AB}=?$$ $$\begin{aligned} \phi_{AB}=&\cos^{-1}\(\frac{\vec A\cdot\vec B}{AB}\)\\ =&\cos^{-1}\(\frac{14.0}{42.0}\)\\ \approx&1.23\ut{rad} \end{aligned}$$