What you did was good
Let's rename the geometric series into S :
Now the text says that and
Hence you have to solve this :
Can you try to get 3a=8d ?
The 1st term of an arithmetic progression is and the common difference is , where .
(i) Write down expressions, in terms of and , for the 5th term and 15th term.
The 1st term, the 5th term and the 15th term of the arithmetic progression are the first terms of a geometric progression.
(ii) Show that .
(iii) Find the common ratio of the geometric progression.
(ii) Arithmetic Progression
I don't know what to do next... Need help!