Practicing Success
If three unit vectors $\vec{a}, \vec{b}, \vec{c}$ satisfy $\vec{a}+\vec{b}+\vec{c}=\vec{0}$, then angle between $\vec{a}$ and $\vec{b}$ is: |
$\frac{\pi}{3}$ $\frac{2 \pi}{3}$ $\frac{\pi}{6}$ $\frac{5 \pi}{6}$ |
$\frac{2 \pi}{3}$ |
$\vec{a}+\vec{b}=-\vec{c}$ $\Rightarrow|\vec{a}+\vec{b}|^2=|\vec{c}|^2=1$ $\Rightarrow|\vec{a}|^2+|\vec{b}|^2+2 \vec{a} . \vec{b}=1$ $\Rightarrow \vec{a} . \vec{b}=-\frac{1}{2}$ $\Rightarrow|\vec{a} \| \vec{b}| \cos \theta=-\frac{1}{2}$ $\Rightarrow \cos \theta=-\frac{1}{2}$ $\Rightarrow \theta=-\frac{2 \pi}{3}$ Hence (2) is correct answer. |