Answer
$\dfrac{\pi}{3}$
Work Step by Step
Formula to calculate the angle between two plane vectors $a$ and $b$ is defined as:
$ \theta = \cos ^{-1} (\dfrac{a \cdot b}{|a||b|})$
Here, $a=\lt 1,1,0 \gt$ and $b=\lt 0,1,1 \gt$
$|a|=\sqrt{1^2+1^2+0^2}= \sqrt {2}$ and $|b|=\sqrt{0^2+1^2+(1)^2}=\sqrt 2$
Thus, $ \theta = \cos ^{-1} (\dfrac{1}{ (\sqrt 2 )(\sqrt 2)})=\cos^{-1} (\dfrac{1}{2})=\dfrac{\pi}{3}$
