Figure 3
\(\mathbf{r}=\mathbf{r}(t), \) \(a\leq t\leq b\)
\(\mathbf{r}=\mathbf{r}(t), \) \(a\leq t\leq b\) \(\mathbf{r}=\mathbf{r}(t), \) \(a\leq t\leq b\) \(\mathbf{r}=\mathbf{r}(t), \) \(a\leq t\leq b\)