Let
a and
R be the first term and common ratio of a GP.
∴Tp=aRP−1=xTq=aRq−1=y And
Tr=aRr−1=z⇒ logx=loga+(p−1)logR logy=loga+(q−1)logR and
logz=loga+(r−1)logR $ \therefore \begin{vmatrix}
\log x & x & 1 \\
\log y & y & 1 \\
\log z & z & 1
\end{vmatrix}= \begin{vmatrix}
\log a + p - 1 & \log R & p & 1 \\
\log a + q - 1 & \log R & q & 1 \\
\log a + r - 1 & \log R & r & 1
\end{vmatrix}$
$ = \begin{vmatrix}
\log a & p & 1 \\
\log a & q & 1 \\
\log a & r & 1
\end{vmatrix} + \begin{vmatrix}
p - 1 & \log R & p & 1 \\
q - 1 & \log R & q & 1 \\
r - 1 & \log R & r & 1
\end{vmatrix}$
$ = \log a \ \ \begin{vmatrix}
1 & p & 1 \\
1 & q & 1 \\
1& r & 1
\end{vmatrix} + \log R \ \ \begin{vmatrix}
p - 1 & p - 1 & 1 \\
q - 1 & q - 1 & 1 \\
r - 1 & r - 1 & 1
\end{vmatrix}$
C2→C2−C3=0+0=0 (
∵ two columns are identical)