(c) If $r$ is the common ratio, then ${a_n} = {a_1}{r^{n - 1}}$ for all $n \ge 1$
$\Rightarrow \log {a_n} = \log {a_1} + (n - 1)\log r$
= $A + (n - 1)R$, where $\log {a_1} = A$ and $\log r = R$.
Thus in $\Delta $, on applying ${C_2} \to {C_2} - {C_1}$ and ${C_3} \to {C_3} - {C_2}$, we obtain ${C_2}$ and ${C_3}$ are identical.
Thus $\Delta = 0$.