Answer
$s = \displaystyle \frac{7\pi}{4}$
Work Step by Step
For any real number $s$ represented by a directed arc on the unit circle,
$\sin s=y\quad \cos s=x \quad \displaystyle \tan s=\frac{y}{x} (x\neq 0)$
Use Figure 13 on page 111.
----------------
In the interval $[\displaystyle \frac{3\pi}{2},2\pi]$, (quadrant IV), we search for
a point (x,y) on the unit circle, such that $\displaystyle \frac{y}{x}=-1$
$(\displaystyle \tan s=\frac{y}{x})$
We find the point$:$
$(\displaystyle \frac{\sqrt{2}}{2},-\frac{\sqrt{2}}{2}) \quad $assigned to $315^{o}$, or $\displaystyle \frac{7\pi}{4}$ rad.
$\displaystyle \tan\frac{7\pi}{4}=\frac{y}{x}=\frac{-\frac{\sqrt{2}}{2}}{\frac{\sqrt{2}}{2}}=-1$
$s = \displaystyle \frac{7\pi}{4}$
