## Algebra 2 (1st Edition)

$\dfrac{\sqrt 2}{2}$
When $P$ is the period of $f(x)$ then we have $f(x+p)=f(x)$ Since the function cosine is positive in the fourth quadrant and $\dfrac{7 \pi}{4}$ is also positive in the same quadrant. Thus, the reference angle is: $\dfrac{\pi}{4}$. Here, we have $\cos (\dfrac{7 \pi}{4})=\cos (\dfrac{\pi}{4})$ and $\cos (\dfrac{\pi}{4})=\dfrac{\sqrt 2}{2}$