Practicing Success
If $\vec a+\vec b+\vec c=\vec 0,|\vec a|= 3,|\vec b|=5$, and $|\vec c|=7$, then the angle between $\vec a$ and $\vec b$ is |
$\frac{π}{2}$ $\frac{π}{4}$ $\frac{π}{6}$ $\frac{π}{3}$ |
$\frac{π}{3}$ |
We have, $\vec a+\vec b+\vec c=\vec 0$ $⇒\vec c=-(\vec a+\vec b)$ $⇒|\vec c|=|-(\vec a+\vec b)|$ $⇒|\vec c|^2=|\vec a+\vec b|^2$ $⇒|\vec c|^2=|\vec a|^2+|\vec b|^2+2(\vec a.\vec b)$ $⇒|\vec c|^2=|\vec a|^2+|\vec b|^2+2|\vec a||\vec b|\cos θ$, where θ is angle between $\vec a$ and $\vec b$. $⇒49=9+25+ 30 \cos θ$ $⇒15=30\cos θ⇒\cos θ=\frac{1}{2}⇒θ=\frac{π}{3}$ |