For #'s 8 and 10, three of the elements a1, an, r, n, and Sn of the geometric progression re given. Find the missing elements in each case.

8. r = -3, n = 5, an = 162

10. a1 = 3, r = 2/5, Sn = 609/125

12. Find the value of k so that 2k+2, 5k-11, and 7k-13 will form a geometric progression.

14. The second term of a geometric progression is 5/4 and its 4th term is 1/5. Find its third term.

16. The terms between any two terms of a geometric progression are called the geometric means between these two terms, Inset four geometric means between 25/4 and 8/125.