Step by step
The question
Question: find for which x the series converges: the sum of (x − 4)ⁿ over 5ⁿ.
1 The ratio
Use the ratio test.
x can be negative, so we take sizes, with bars.
Divide the next term by this one.
(x − 4) to the n + 1, over (x − 4) to the n, leaves one (x − 4).
The powers of 5 leave 1/5.
2 The limit
So L is 1/5 times the size of x minus 4.
3 The radius
It converges when L is less than 1.
Multiply both sides by 5.
So the radius of convergence is 5: x can be up to 5 away from 4.
Careful!
Not 1/5!
L is 1/5 times the size, so the size must be less than 5.
Open up the bars: x is between −1 and 9.
4 The ends
At x = 9: (x − 4) is 5, and its powers cancel the powers of 5.
The terms do not go to 0: it diverges.
At x = −1: the same, but with signs that take turns.
The terms do not go to 0: it diverges.
5 The interval
In interval notation: (−1, 9).
A square bracket means the end is in; a round one, it is not.
6 Check
Let's check an x inside, 6.5, with a calculator: 40 terms and 80 terms add up to almost the same, 1.
It converges there.
Right!
Let's look back.
First: the ratio.
Then: the limit.
Then: the radius.
Then: the ends.
Then: the interval.
Last: check.