Answer
positive; I; $30^{\circ}$; $\frac{\sqrt 3}{2}$
Work Step by Step
$390^{\circ}$ is coterminal with $390^{\circ}-360^{\circ}=30^{\circ}$.
$30^{\circ}$ is in the first quadrant.
In the first quadrant, the value of $\cos\theta$ is positive.
For the first quadrant, reference angle $\theta'=\theta=30^{\circ}$
$\cos 390^{\circ}=\cos 30^{\circ}=\frac{\sqrt 3}{2}$
