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Use the Ratio Test to determine whether the series is convergent or divergent.

sum_(k=1)^(oo)9ke^(-k)
Identify 
a_(k).
Evaluate the following limit.

lim_(k rarr oo)|(a_(k)+1)/(a_(k))|

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Since 
lim_(k rarr oo)|(a_(k)+1)/(a_(k))|?vv1,- Select--

Use the Ratio Test to determine whether the series is convergent or divergent.\newlinek=19kek \sum_{k=1}^{\infty} 9 k e^{-k} \newlineIdentify ak a_{k} .\newlineEvaluate the following limit.\newlinelimkak+1ak \lim _{k \rightarrow \infty}\left|\frac{a_{k}+1}{a_{k}}\right| \newline \square \newlineSince limkak+1ak?1, \lim _{k \rightarrow \infty}\left|\frac{a_{k}+1}{a_{k}}\right| ? \vee 1,- Select--

Full solution

Q. Use the Ratio Test to determine whether the series is convergent or divergent.\newlinek=19kek \sum_{k=1}^{\infty} 9 k e^{-k} \newlineIdentify ak a_{k} .\newlineEvaluate the following limit.\newlinelimkak+1ak \lim _{k \rightarrow \infty}\left|\frac{a_{k}+1}{a_{k}}\right| \newline \square \newlineSince limkak+1ak?1, \lim _{k \rightarrow \infty}\left|\frac{a_{k}+1}{a_{k}}\right| ? \vee 1,- Select--
  1. Identify General Term: Identify aka_{k} as the general term of the series, which is 9kek9ke^{-k}.
  2. Evaluate Limit: Evaluate the limit limkak+1ak\lim_{k \to \infty}\left|\frac{a_{k}+1}{a_{k}}\right|. Start by finding ak+1a_{k}+1 which is 9(k+1)e(k+1)9(k+1)e^{-(k+1)}.
  3. Calculate Ratio: Now, calculate ak+1ak=9(k+1)e(k+1)9kek=k+1ke1\frac{a_{k}+1}{a_{k}} = \frac{9(k+1)e^{-(k+1)}}{9ke^{-k}} = \frac{k+1}{k}e^{-1}.
  4. Simplify Expression: Simplify (k+1k)e1\left(\frac{k+1}{k}\right)e^{-1} to (1+1k)e1\left(1 + \frac{1}{k}\right)e^{-1}.
  5. Take Limit: Take the limit limk(1+1k)e1\lim_{k \to \infty}(1 + \frac{1}{k})e^{-1}. As kk approaches infinity, 1k\frac{1}{k} approaches 00, so the expression simplifies to e1e^{-1}.
  6. Apply Ratio Test: Since e1e^{-1} is less than 11, the Ratio Test tells us that the series is convergent.

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