Chapter 3 - Radian Measure and the Unit Circle - Section 3.3 The Unit Circle and Circular Functions - 3.3 Exercises - Page 123: 6

$\frac{\pi}{3}$

As $\cos s=x$, From the given data, we understand that some angle $s$ in the first quadrant intersects the unit circle at the point with x-coordinate equal to $\frac{1}{2}$. From the solution of excercise 1, we know that this angle is $\frac{\pi}{3}$.