Answer
Result True.
Work Step by Step
The statement is True. From a theorem in an earlier section, we know that if a vector-valued function $\mathbf{u}$ has a constant norm, then $\mathbf{u}$ and $\mathbf{u'}$ are orthogonal. Then in a later theorem, we saw for $\mathbf{r}(s)$ where $s$ is an arc length parameter that $\left\|\mathbf{r'}(s)\right\| = 1$, that is $\mathbf{r'}$ has a constant norm of $1$. Then we can conclude that its derivative, $\mathbf{r''}$, if it is defined, is orthogonal to $\mathbf{r'}$. Result True.
