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Unit 10

10.3 The nth Term Test for Divergence

1 min read•june 18, 2024

10.3 The nth Term Test for Divergence

Welcome back to Unit 10 of AP Calculus BC! Today, we’re going to discuss the nth-term test for divergence with series. Let’s get started!

🤷‍♀️ What is the nth Term Test for Divergence?

As the name suggests the nth Divergence test tells us if a series will diverge! (mind-blowing stuff guys, I know 🤯). The Divergence test states that:

if lim⁡n→∞an≠0,∑an diverges\text{if} \ \displaystyle{\lim_{n \to \infty}} a_n \not = 0, \sum a_n \ \text{diverges}

As we can see, if the nth term doesn't approach 0, the series diverges. On the other hand, if the nth term approaches 0, it creates a situation where the series might converge or still diverge. The crucial point here is that the fate of the series hinges on whether the nth term tends towards zero or not.

🤓 Divergence Test Walkthrough

Let’s try a practice problem together! There are really only 3 steps involved with this:

  1. ✏️ Convert to limit notation.
  2. 📏 Evaluate the limit.
  3. 🤔 Make your conclusion based on the nth-term test.

Determine if the series ana_n diverges.

∑n=1∞5n−1n−2\sum_{n=1}^∞ \dfrac{5n -1}{n-2}

✏️ Step 1: Convert to limit notation.

lim⁡n→∞5n−1n−2\lim_{n \to \infty} \dfrac{5n -1}{n-2}

📏 Step 2: Evaluate the limit.

lim⁡n→∞n(5−1n)n(1−2n)\lim_{n \to \infty} \dfrac{n(5- \frac{1}{n} )}{n(1-\frac{2}{n})}
5−1∞1−2∞=51\dfrac{5- \frac{1}{\infty} }{1-\frac{2}{\infty}} = \frac{5}{1}

Recall that any number divided by ∞\infty is 0.

lim⁡n→∞n(5−1n)n(1−2n)=51≠0\lim_{n \to \infty} \dfrac{n(5- \frac{1}{n} )}{n(1-\frac{2}{n})} = \frac{5}{1} \neq 0
∴ ∑n=1∞5n−1n−2  diverges\therefore \ \sum_{n=1}^∞ \dfrac{5n -1}{n-2} \ \ \text{diverges}

Not too bad, right? We’re mainly just applying a new test to the mathematics that we are already familiar with!

📝 Divergence Test Practice Problems

Try the following two practice questions yourself!

1. ∑n=1∞n3+3n2n3−51. \ \sum_{n=1}^∞ \dfrac{n^3+3n}{2n^3-5}
2. ∑n=1∞arctan⁡(n)2. \ \sum_{n=1}^∞ \arctan(n)

✅ Divergence Test: Solution 1

Remember the three steps involved and the nth-term test itself.

lim⁡n→∞n3+3n2n3−5\lim_{n \to \infty} \dfrac{n^3+3n}{2n^3-5}
lim⁡n→∞n3(1+3n2)n3(2−5n3)\lim_{n \to \infty} \dfrac{n^3(1+\dfrac{3}{n^2})}{n^3(2-\dfrac{5}{n^3})}
1+3∞2−5∞=12\dfrac{1+\dfrac{3}{\infty}}{2-\dfrac{5}{\infty}} = \dfrac{1}{2}
∴ ∑n=1∞n3+3n2n3−5  diverges\therefore \ \sum_{n=1}^∞ \dfrac{n^3+3n}{2n^3-5} \ \ \text{diverges}

Great work!

✅ Divergence Test: Solution 2

Last question 🎉

lim⁡n→∞arctan⁡(n)\lim_{n \to \infty} \arctan(n)

Untitled

Image courtesy of Math.net

As arctan goes to ∞\infty, it stays at π2\dfrac{\pi}{2}.

 lim⁡n→∞arctan⁡(n)=π2 \ \lim_{n \to \infty} \arctan(n) = \dfrac{\pi}{2}
∴ ∑n=1∞arctan⁡(n) diverges\therefore \ \sum_{n=1}^∞ \arctan(n) \ \text{diverges}

🕺 Closing

In conclusion, the nth Term Test for Divergence is a powerful tool for determining whether a series diverges. Remember, if the limit of the nth term does not approach zero, the series diverges. However, passing the divergence test doesn't provide information about convergence. Good luck! 🍀

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About Us
About FiveableBlogCareersCode of ConductTerms of UsePrivacy PolicyCCPA Privacy Policy
Resources
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Student Wellness
Wellness GuidesJoon Care ResourcesCrisis Text LineInternational Hotlines

© 2024 Fiveable Inc. All rights reserved.