Chapter 12.3, Problem 1QCE

If $\text{r}\left(t\right)$ is a smooth vector-valued function, then the integral ${\displaystyle {\int }_a^b‖\frac{\text{dr}}{dt}‖}dt$ may be interpreted geometrically as the $\text{\_\_\_\_\_\_\_\_\_\_\_\_\_}\text{.}$