If $\vec a$ and $\vec b$ are unit vectors inclined to x-axis at angles 30° and 120°, then $|\vec a +\vec b|$ equals

$\sqrt{2/3}$ $\sqrt{2}$ $\sqrt{3}$ 2

$\sqrt{2}$

Clearly, $\vec a$ and $\vec b$ are at right angle. $∴|\vec a+\vec b|^2=|\vec a|^2+|\vec b|^2+2|\vec a||\vec b|\cos 90°$ $⇒|\vec a+\vec b|^2=1+1+0=2⇒|\vec a+\vec b|=\sqrt{2}$