$ = {{\rm{a}}^{3{\rm{k}}}} \cdot {{\rm{r}}^{6k}} \cdot {{\rm{a}}^3}{{\rm{r}}^3}\left| {\begin{array}{*{20}{c}}
1&1&1\\
1&{\rm{r}}&{{{\rm{r}}^2}}\\
1&{{{\rm{r}}^2}}&{{{\rm{r}}^4}}
\end{array}} \right|$
$=a^{3(k+1)} \cdot r^{6 k+3}(1-r)\left(r-r^{2}\right)\left(r^{2}-1\right)$
clearly, $\mathrm{k}=-1$
$\therefore \Delta=r^{-2}(1-r)^{2}\left(r^{2}-1\right)=(r-1)^{2}\left(1-\frac{1}{r^{2}}\right)$