If 4th, 10th and 16th terms of a GP. are x, y and z respectively, Prove that y, z are in GP.

Let a be the first term and r be the common ratio of G.P.
Given
                                                                ...(i)
                                                                    ...(ii)
                                                                  ...(iii)
Multiply (i) and (iii), we get
or                           or                     [By using (ii)]

∴                               or            or  x, y, z are in G.P.