Question:

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.

Attempt:

(i)

(ii) Arithmetic Progression

Geometric Progression

I don't know what to do next... Need help!