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Math Help - Power Series Question

  1. #1
    Junior Member
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    Power Series Question

    Power series from 1 to infinity of (n^n)*x^n? I applied the ratio test:
    ((n+1)^(n+1)x^(n+1))/((n^n)*x^n) = ((n+1)^(n+1)*x)/(n^n) and after taking the limit of n->infinity I got nx which goe s to infinity. I think I messed up, any help?
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  2. #2
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    Hello, thehollow89!

    The algebra of the Ratio Test is very tricky . . .


     \sum^{\infty}_{n=1} n^nx^n

    We have: . R \;=\;\frac{a_{n+1}}{a_n} \;=\;\frac{(n+1)^{n+1}x^{n+1}} {n^nx^n} \;=\;\frac{(n+1)(n+1)^n}{n^n}\,x<br />

    . . . . . . . . . = \;(n+1)\left(\frac{n+1}{n}\right)^n\!\!x \;=\;(n+1)\left(1 + \frac{1}{n}\right)^n\!\!x


    \text{Take the limit: }\:\lim_{n\to\infty}\left[(n+1)\left(1 + \frac{1}{n}\right)^n x\right]

    . . . . . . . . =\;\underbrace{\left[\lim_{n\to\infty}(n+1)\right]}_{\text{This is }\infty}\cdot\underbrace{\left[\lim_{n\to\infty}\left(1 + \frac{1}{n}\right)^n\,\right]}_{\text{This is }e}\cdot x

    . . . . . . . . =\;\; \infty\cdot e\cdot x \;\;=\;\;\infty\quad\hdots\quad\text{The series diverges.}

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