Figure 13 
\(\mathbf{r}( t) =R\;\cos t\mathbf{i}+R\;\sin t\mathbf{j,}\;0\leq t\leq 2\pi \)
\(\mathbf{r}( t) =R\;\cos t\mathbf{i}+R\;\sin t\mathbf{j,}\;0\leq t\leq 2\pi \) \(\mathbf{r}( t) =R\;\cos t\mathbf{i}+R\;\sin t\mathbf{j,}\;0\leq t\leq 2\pi \)