If `p^{th}`, `q^{th}` and `r^{th}` terms of a geometric series are `a`, `b`, `c` respectively, then show that $ \begin{vmatrix} \log a & p & 1 \\ \log b & q & 1 \\ \log c & r & 1 \end{vmatrix} = 0 $ where `a, b, c` are positive.