Question

Test the following series for convergence,

(i) \sum_{n=1}^{\infty }n\,x^{n-1},x>0.

(ii) \sum_{n=1}^{\infty }\left [ \sqrt{n^4+9}-\sqrt{n^4-9} \right ]

12 Mar 2024
Answer :
Word Count : 414

Part (i)
Consider the series
\[
\sum_{n=1}^{\infty} n \, x^{n-1}.
\]

We can recognize this as a geometric series, with each term being \(n \, x^{n-1}\). By manipulating the original series, we can derive a formula to test its convergence.

Given a geometric series of the form \(\sum_{n=0}^{\infty} a r^n\), it converges if \(|r| < 1\). Similarly, the series provided can be rewritten in a simpler form by noting that it is an infinite series starting at \(n ______ ___ ___ _____ ______.
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