Separate the sum of 2n terms in two sums:

   First sum S1 consists of odd terms of the original sequence, and 

   second sum S2 consists of even terms.


Then the terms of S1 form geometric progression with the first term 1 and the common ratio of ac (= a*c).


The terms of S2 form geometric progression with the first term "a" and the common ratio of ac (= a*c).


Calculate the sums S1 and S2 using the common rule (formula) for geometric progression.


Then take the sum S1 + S2.