$\overline{\mathrm{a}}, \overline{\mathrm{b}}$ and $\overline{\mathrm{c}}$ are three vectors such that…
$\overline{\mathrm{a}}, \overline{\mathrm{b}}$ and $\overline{\mathrm{c}}$ are three vectors such that $\overline{\mathrm{a}}+\overline{\mathrm{b}}+\overline{\mathrm{c}}=\overline{0}$ and $|\overline{\mathrm{a}}|=3,|\overline{\mathrm{b}}|=5,|\overline{\mathrm{c}}|=7$, then the angle between $\overline{\mathrm{a}}$ and $\overline{\mathrm{b}}$ is
$\frac{\pi}{4}$
$\frac{\pi}{2}$
$\frac{\pi}{3}$
$\frac{\pi}{6}$
Solution
$\overline{\mathrm{a}}+\overline{\mathrm{b}}+\overline{\mathrm{c}}=0 \Rightarrow \overline{\mathrm{c}}=-(\overline{\mathrm{a}}+\overline{\mathrm{b}})$ and let angle between $\overline{\mathrm{a}}$ and $\overline{\mathrm{b}}$ be $\theta$
$\begin{aligned}
& \therefore|\overline{\mathrm{c}}|^2=(\overline{\mathrm{a}}+\overline{\mathrm{b}})^2=|\overline{\mathrm{a}}|^2+|\overline{\mathrm{b}}|^2+2 \overline{\mathrm{a}} \cdot \overline{\mathrm{b}} \\
& =|\overline{\mathrm{a}}|^2+|\overline{\mathrm{b}}|^2+2|\overline{\mathrm{a}}| \cdot|\overline{\mathrm{b}}| \cdot \cos \theta \\
& \therefore(7)^2=(3)^2+(5)^2+2(3)(5) \cos \theta \\
& \therefore 49=9+25+30 \cos \theta \Rightarrow \cos \theta=\frac{15}{30}=\frac{1}{2} \Rightarrow \theta=\frac{\pi}{3}
\end{aligned}$