Chapter 6 - Analytic Trigonometry - Section 6.2 The Inverse Trigonometric Functions (Continued) - 6.2 Assess Your Understanding - Page 480: 13

$2$

The range of $\cos^{-1}{x}$ is $[0, \pi]$. Here, $\cos^{-1}{(\dfrac{1}{2})}=\dfrac{ \pi}{3}$, because $\cos{(\dfrac{\pi}{3})}=-\dfrac{1}{2}$ and $\dfrac{\pi}{3}$ lies in the range of $\cos^{-1}{x}$ . Therefore, we have: $\sec [\cos^{-1}{(\dfrac{1}{2})}]=\sec ({\dfrac{ \pi}{3}})=2$