Chapter 4 - Section 4.1 - Angles and Radian Measure - Concept and Vocabulary Check - Page 532: 8

$r\theta$

RECALL: The length of a circular arc ($s$) is given by the formula $s = r\theta$, where r =radius and $\theta$ is the measure of the central angle. Thus, the missing expression in the given statements is: $r\theta$