Answer
$$\cos \theta=0.$$
That is $\theta=\frac{\pi}{2}$.
Work Step by Step
Let $u=(2,-1),\quad v=(\frac{1}{2},1), \quad =u\cdot v
$The angle $\theta$ between $u$ and $v$ is given by the formula
$$\cos \theta=\frac{}{\|u\| \cdot\|v\|}=\frac{1-1}{\sqrt{ 4+1}\sqrt{ \frac{1}{4}+1}}=0.$$
That is $\theta=\frac{\pi}{2}$.
