Step by step
The question
Question: decide if the series converges: the sum of 2ⁿ over n³ for n from 1 to infinity.
1 The next term
The ratio test: divide each term by the one before it, and let n grow.
If the limit L is less than 1, the terms shrink like a geometric series: it converges.
Bigger than 1: it diverges.
Exactly 1: the test cannot tell.
The next term: put n + 1 wherever there is an n.
2 The ratio
Dividing by aₙ is the same as times its flip.
Group the same kinds together.
2 to the n + 1, over 2 to the n, is just 2.
3 The limit
As n grows: n + 1/n goes to 1.
So L = 2.
4 Decide
L = 2 is bigger than 1: the series diverges.
Careful!
L = 1 tells us nothing.
Only less than 1 converges, and more than 1 diverges.
5 Check
Let's check with a calculator: 5, 10, 20 terms add up to 3.3023, 6.1981, 324.6536.
It grows without end.
Right!
Let's look back.
First: the next term.
Then: the ratio.
Then: the limit.
Then: decide.
Last: check.