Let $\mathbf{r}_{1}=\left\langle x_{1}, y_{1}\right\rangle, \mathbf{r}_{2}=\left\langle x_{2}, y_{2}\right\rangle$, and $\mathbf{r}=\langle x, y\rangle .$ As-
suming that $k>\left\|\mathbf{r}_{2}-\mathbf{r}_{1}\right\|$, describe the set of all points $(x, y)$ for which $\left\|\mathbf{r}-\mathbf{r}_{1}\right\|+\left\|\mathbf{r}-\mathbf{r}_{2}\right\|=k$