Anyone help? Math infinite series Which of the following infinite geometric sequences will have a related convergent series?

{16, 20.8, 27.04, 35.15,…}

{44, 44, 44, 44, …}

{5, 6, 7.2, 8.64,…}

(120, 96, 76.8, 61.44...)

Explanation:

A sequence is simply a list of values. There may or may not be a pattern to the list, though usually in a math class your teacher would use a pattern.

A series is where we add partial sums of a certain sequence to form a new sequence. For example, let's say A = {44, 44, 44, ...} is one sequence and we could form a list of partial sums to be B = {44, 88, 132, ...} The 88 is from adding the first two terms of sequence A together. The 132 is from adding the first three terms of sequence A. This process is continued forever.

For a series to be convergent, the terms must approach some finite single value. They likely won't actually reach this value but they should get closer and closer (think of it like an asymptote). When I formed series B shown above, note how the terms keep growing. It turns out they won't stop growing either and that means the series diverges to positive infinity. So we rule out choice B. The same happens for choices A and C as well.

To make sure the series approaches some finite value, the underlying sequence its based on must have the terms approaching 0. Choice D seems to have this happen. The terms of the sequence are getting smaller and smaller. Adding on smaller pieces will produce terms in a series that would likely get us closer to some finite target.

Keep in mind that not all decreasing sequences have a converging series. A famous example would be the harmonic sequence {1, 1/2, 1/3, 1/4, ...} which its series does not converge. The proof of this lengthy so I'll leave it out.

In the case of choice D, it is a geometric sequence with a = 120 as the starting term and r = 0.8 as the common ratio. The series converges to

S = a/(1-r) = 120/(1-0.8) = 600

you need a common difference between consecutive terms to be constant, like 1,2,3,4 or 2,4,6,8, or 1,3,6,9