2020-II-35

$\begin{array}{cl}
& \log a – \log a^{-3} \\
= & \log a +3\log a \\
= & 4\log a
\end{array}$

$\begin{array}{cl}
& \log a^5 – \log a \\
= & 5 \log a – \log a \\
= & 4\log a
\end{array}$

II 為等差數列。

$\begin{array}{cl}
& (9-5a) – (8-4a) \\
= & 1 – a
\end{array}$

$\begin{array}{cl}
& (10-6a) – (9-5a) \\
= & 1- a
\end{array}$

III 為等差數列。

$\begin{array}{cl}
& \cos 90^\circ – \cos (90-a)^\circ \\
= & -\sin a^\circ
\end{array}$

$\begin{array}{cl}
& \cos (90+a)^\circ – \cos 90^\circ \\
= & – \sin a^\circ
\end{array}$