What is the 7th term in the geometric sequence in which a10 is 9 and a13 is -72?
a. 2b. - 1/8c. - 9/8d. 1/4

Recall, a geometric series has this form with constants A for n=0, and k, the ratio of two consecutive terms n/(n+1)


you are given two values, for n=13 and n=10, put both into the equation




Two equations with two unknown values A and k, you can solve for those.

Solving both for A,





The general equation for the series is then,


use n=7

Hold on lemme do the work