Computing number of terms in a geometric series if 'a' and 'r' are known

Question

How do you compute the number of terms in a geometric series if the ratio and first term are known ? Also will there be any error in prediction of number of terms rather than computing the actual number of terms by iteration ? 

Answer 1

Question 1: To know that it is geometric at least three terms must have been given. The common ratio can be determined.

Question 2: Would you rather your teacher calculate you grade or predict it?

Answer 2

A geometric series can be of finite duration (fixed at n terms) or it can be infinite. The context of a problem will generally make it clear. I think prediction is not applicable.