Step 1: Formula / Definition}
\[
a, b, c \text{ in GP} \Rightarrow b^2 = ac
\]
Step 2: Calculation / Simplification}
Take \(\log\): \(\log(b^2) = \log(ac)\)
\(2\log b = \log a + \log c\)
\(\log b = \frac{\log a + \log c}{2}\)
\(\therefore \log a, \log b, \log c\) are in AP.
Step 3: Final Answer
\[
\text{AP}
\]