How to find number of terms in a geometric progression

Question

What is the equation two find the number of terms in the geometric progression given first two and the last term?

Answer 1

Let $a_1,a_2$ be the first two terms and let $a_f$ be the final term, then the number of terms is $\log_{|a_2/a_1|}(|a_f/a_1|)+1$.