證明等比數列

由 loga , log b , log c 為等差 有

log a - log b = log b - log c

log(a/b) = log (b/c)

a/b = b/c (s)

根a^3 / 根b^3 = (a/b)^3/2

根b^3 / 根c^3 = (b/c)^3/2

由(s) 可得 (a/b)^3/2 = (b/c)^3/2

所以它們是等比數列