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# 9.5 Testing Convergence at Endpoints

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9.5 Testing Convergence at Endpoints
The original Hanford High School, Hanford, Washington Greg Kelly, Hanford High School, Richland, Washington

Remember: The Ratio Test: If is a series with positive terms and then: The series converges if The series diverges if The test is inconclusive if

This section in the book presents several other tests or techniques to test for convergence, and discusses some specific convergent and divergent series.

Nth Root Test: If is a series with positive terms and then: The series converges if Note that the rules are the same as for the Ratio Test. The series diverges if The test is inconclusive if

example: ?

formula #104 formula #103 Indeterminate, so we use L’Hôpital’s Rule

example: ? it converges

another example: it diverges

Remember that when we first studied integrals, we used a summation of rectangles to approximate the area under a curve: This leads to: The Integral Test If is a positive sequence and where is a continuous, positive decreasing function, then: and both converge or both diverge.

Example 1: Does converge? Since the integral converges, the series must converge. (but not necessarily to 2.)

p-series Test converges if , diverges if .
We could show this with the integral test. If this test seems backward after the ratio and nth root tests, remember that larger values of p would make the denominators increase faster and the terms decrease faster.

the harmonic series: diverges. (It is a p-series with p=1.) It diverges very slowly, but it diverges. Because the p-series is so easy to evaluate, we use it to compare to other series.

Limit Comparison Test If and for all (N a positive integer)
If , then both and converge or both diverge. If , then converges if converges. If , then diverges if diverges.

Example 3a: When n is large, the function behaves like: harmonic series Since diverges, the series diverges.

Example 3b: When n is large, the function behaves like: geometric series Since converges, the series converges.

Good news! Alternating Series Test Alternating Series
The signs of the terms alternate. If the absolute values of the terms approach zero, then an alternating series will always converge! Alternating Series Test Good news! example: This series converges (by the Alternating Series Test.) This series is convergent, but not absolutely convergent. Therefore we say that it is conditionally convergent.

Alternating Series Estimation Theorem
Since each term of a convergent alternating series moves the partial sum a little closer to the limit: Alternating Series Estimation Theorem For a convergent alternating series, the truncation error is less than the first missing term, and is the same sign as that term. This is a good tool to remember, because it is easier than the LaGrange Error Bound.

There is a flow chart on page 505 that might be helpful for deciding in what order to do which test. Mostly this just takes practice. To do summations on the TI-89: becomes F3 4 becomes

To graph the partial sums, we can use sequence mode.
4 ENTER Y= ENTER WINDOW GRAPH

To graph the partial sums, we can use sequence mode.
4 ENTER Y= ENTER WINDOW GRAPH Table

p To graph the partial sums, we can use sequence mode. Graph……. 4 Y=
ENTER Y= ENTER WINDOW GRAPH Table p

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