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Test the following series for convergence.

2.∑n=1∞(−2)nn2

Short Answer

Expert verified

The series∑n=1∞(−2)nn2 diverges.

Step by step solution

01

Given information

It is given that the series is∑n=1∞(−2)nn2

02

Convergence Test

According to the convergence test for alternating series: An alternating series converges if the absolute value of the terms decreases steadily to zero, that is, if an+1≤an, and limn→∞an=0.

03

Apply the properties of convergence test

Let an=(−2)nn2.

Then an+1=(2)n+1(n+1)2.

Apply the convergence test as an+1≤an.

To check use the following method:

If an+1an<1, that means an>an+1.

And, if an+1an>1, that means an<an+1. Thus:

an+1an=(2)n+1(n+1)2(2)nn2=n2(2)n+1(n+1)2(2)n=2n2n2+2n+1

If 2n2−n2+2n+1<0, then 2n2n2+2n+1<1.

2n2−n2+2n+1=n2−2n−1

For n≥3, n2−2n−1>0.

That means an+1an=(2)n+1(n+1)2(2)nn2>1for n≥3.

Since, the first property is not satisfied, so the series diverges.

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