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∑n=1∞(-1)nn3xn

Short Answer

Expert verified

The interval of convergence is-1,1

Step by step solution

01

Given Information  

The power series is∑n=1∞-1nn3xn

02

Definition of the interval of convergence. 

The Interval of convergence is the interval in which the power series is convergent.

03

Find the interval. 

The power series is ∑n=1∞-1nn3xn

Let pn=an-1an.

Substitute the value of the power series in the formula above, the equation becomes as follows,

ÒÏn=an+1an

=(n+1)3xn+1n3xn

=(n+1)3xn3

Apply limits in the above equation,

ÒÏlimn→∞=1+1n3x

=x

The power series is convergent for ÒÏ<1,

Hence the given power series is convergent for x<1and divergent for x>1.

Now check the ends points,

When x=1series is ∑n=1∞-1nn3, which is a divergent alternating series.

When x=-1series is∑n=1∞n3, which is a divergent series.

Hence the interval of convergence is -1,1.

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