Since we are given that
log(a) + log(b) + log(c) + ... is an arithmetic progression,
then log(b)-log(a) = log(c)-log(b) = ... = the common difference
Therefore by a principle of logs, log(b/a) = log(c/d) = ...
Therefore b/a = c/d = ... = common ratio of a,b,c,...
Therefor a,b,c,... is a geometric progression.
Edwin