Determining the convergence of $ sumlimits_{n=1}^{infty}frac{-(n-1)}{nsqrt{n+1}}$
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We have to determin the convergence of the following sum:
begin{equation}
sumlimits_{n=1}^{infty}frac{-(n-1)}{nsqrt{n+1}}
end{equation}
And we've seen multiple ways to determine if the sum converges or diverges, but they only work with non negative terms. So tests like d'Alemberts ratio test, Cauchy's root test or Raabe's test wont work. How would I go about determining the convergence of this sum?
calculus convergence summation
$endgroup$
add a comment |
$begingroup$
We have to determin the convergence of the following sum:
begin{equation}
sumlimits_{n=1}^{infty}frac{-(n-1)}{nsqrt{n+1}}
end{equation}
And we've seen multiple ways to determine if the sum converges or diverges, but they only work with non negative terms. So tests like d'Alemberts ratio test, Cauchy's root test or Raabe's test wont work. How would I go about determining the convergence of this sum?
calculus convergence summation
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$begingroup$
then get rid of the $-1$, since $sum frac{1}{n^{3/2}}$ converges. so you have $sum_n frac{1}{sqrt{n}}$, which diverges
$endgroup$
– mathworker21
Jan 7 at 16:54
$begingroup$
@CalvinGodfrey I don't think that's "without loss of generality"
$endgroup$
– mathworker21
Jan 7 at 16:55
add a comment |
$begingroup$
We have to determin the convergence of the following sum:
begin{equation}
sumlimits_{n=1}^{infty}frac{-(n-1)}{nsqrt{n+1}}
end{equation}
And we've seen multiple ways to determine if the sum converges or diverges, but they only work with non negative terms. So tests like d'Alemberts ratio test, Cauchy's root test or Raabe's test wont work. How would I go about determining the convergence of this sum?
calculus convergence summation
$endgroup$
We have to determin the convergence of the following sum:
begin{equation}
sumlimits_{n=1}^{infty}frac{-(n-1)}{nsqrt{n+1}}
end{equation}
And we've seen multiple ways to determine if the sum converges or diverges, but they only work with non negative terms. So tests like d'Alemberts ratio test, Cauchy's root test or Raabe's test wont work. How would I go about determining the convergence of this sum?
calculus convergence summation
calculus convergence summation
edited Jan 8 at 2:23
Key Flex
7,78461232
7,78461232
asked Jan 7 at 16:51
ViktorViktor
1389
1389
$begingroup$
then get rid of the $-1$, since $sum frac{1}{n^{3/2}}$ converges. so you have $sum_n frac{1}{sqrt{n}}$, which diverges
$endgroup$
– mathworker21
Jan 7 at 16:54
$begingroup$
@CalvinGodfrey I don't think that's "without loss of generality"
$endgroup$
– mathworker21
Jan 7 at 16:55
add a comment |
$begingroup$
then get rid of the $-1$, since $sum frac{1}{n^{3/2}}$ converges. so you have $sum_n frac{1}{sqrt{n}}$, which diverges
$endgroup$
– mathworker21
Jan 7 at 16:54
$begingroup$
@CalvinGodfrey I don't think that's "without loss of generality"
$endgroup$
– mathworker21
Jan 7 at 16:55
$begingroup$
then get rid of the $-1$, since $sum frac{1}{n^{3/2}}$ converges. so you have $sum_n frac{1}{sqrt{n}}$, which diverges
$endgroup$
– mathworker21
Jan 7 at 16:54
$begingroup$
then get rid of the $-1$, since $sum frac{1}{n^{3/2}}$ converges. so you have $sum_n frac{1}{sqrt{n}}$, which diverges
$endgroup$
– mathworker21
Jan 7 at 16:54
$begingroup$
@CalvinGodfrey I don't think that's "without loss of generality"
$endgroup$
– mathworker21
Jan 7 at 16:55
$begingroup$
@CalvinGodfrey I don't think that's "without loss of generality"
$endgroup$
– mathworker21
Jan 7 at 16:55
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
You can use Integral test:
$$f(n)=dfrac{n-1}{nsqrt{n+1}}$$
When you plot a graph, you will notice that $f(n)$ is positive, continuous and decreasing from $n=4$
So, $$int_4^infty dfrac{n-1}{nsqrt{n+1}}=mbox{diverges}$$
So, by integral test, the series diverges
$endgroup$
$begingroup$
I have 2 questions about this: first, why did you take out the negative sign? Can you take out the negative sign and when you do this and the remaining part converges or diverges, does it still converge or diverges, and second why can you start at 4 and not at 1 in your integral?
$endgroup$
– Viktor
Jan 7 at 16:59
1
$begingroup$
@Viktor first, I took the constant out, here $(-1)$, and then performed the test. This doesn't change the result. I took the boundary from $4$ only after checking if $f(n)$ is positive, continuous and decreasing.
$endgroup$
– Key Flex
Jan 7 at 17:03
2
$begingroup$
Okay thanks i think i understand now!
$endgroup$
– Viktor
Jan 7 at 17:03
add a comment |
$begingroup$
For $n>2$,
$$frac{n-1}{nsqrt{n+1}}>frac1{2sqrt{n+1}} $$
As $sum frac1{2sqrt{n+1}}$ diverges. so does $sumfrac{n-1}{nsqrt{n+1}}$ and also $sumfrac{-(n-1)}{nsqrt{n+1}}$.
$endgroup$
add a comment |
$begingroup$
You may also use the comparison test.
Intuitively, as $n$ gets larger, $n-1$ is essentially $n$, and $sqrt{n+1}$ is essentially $sqrt{n}$. That means you can compare $frac{n-1}{nsqrt{n+1}}$ with $frac{n}{nsqrt{n}}=frac{1}{sqrt{n}}$
Formally, you can find a constant $C>0$ such that:
$$frac{n-1}{nsqrt{n+1}}>Cfrac{1}{sqrt{n}}, text{ for }ngeq 2$$
(what could that constant be?)
Now since $sum_{n=1}^inftyfrac{1}{sqrt{n}}$ diverges, that means $sum_{n=1}^inftyfrac{n-1}{nsqrt{n+1}}$ diverges.
Obs.: For any non-zero constant $lambda$, we have that $sum_{n=1}^infty lambda, a_n$ converges $Leftrightarrow sum_{n=1}^infty a_n$ converges (this should be a simple exercise). In particular, the negative sign in $frac{-(n-1)}{nsqrt{n+1}}$ is not relevant to convergence).
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add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
You can use Integral test:
$$f(n)=dfrac{n-1}{nsqrt{n+1}}$$
When you plot a graph, you will notice that $f(n)$ is positive, continuous and decreasing from $n=4$
So, $$int_4^infty dfrac{n-1}{nsqrt{n+1}}=mbox{diverges}$$
So, by integral test, the series diverges
$endgroup$
$begingroup$
I have 2 questions about this: first, why did you take out the negative sign? Can you take out the negative sign and when you do this and the remaining part converges or diverges, does it still converge or diverges, and second why can you start at 4 and not at 1 in your integral?
$endgroup$
– Viktor
Jan 7 at 16:59
1
$begingroup$
@Viktor first, I took the constant out, here $(-1)$, and then performed the test. This doesn't change the result. I took the boundary from $4$ only after checking if $f(n)$ is positive, continuous and decreasing.
$endgroup$
– Key Flex
Jan 7 at 17:03
2
$begingroup$
Okay thanks i think i understand now!
$endgroup$
– Viktor
Jan 7 at 17:03
add a comment |
$begingroup$
You can use Integral test:
$$f(n)=dfrac{n-1}{nsqrt{n+1}}$$
When you plot a graph, you will notice that $f(n)$ is positive, continuous and decreasing from $n=4$
So, $$int_4^infty dfrac{n-1}{nsqrt{n+1}}=mbox{diverges}$$
So, by integral test, the series diverges
$endgroup$
$begingroup$
I have 2 questions about this: first, why did you take out the negative sign? Can you take out the negative sign and when you do this and the remaining part converges or diverges, does it still converge or diverges, and second why can you start at 4 and not at 1 in your integral?
$endgroup$
– Viktor
Jan 7 at 16:59
1
$begingroup$
@Viktor first, I took the constant out, here $(-1)$, and then performed the test. This doesn't change the result. I took the boundary from $4$ only after checking if $f(n)$ is positive, continuous and decreasing.
$endgroup$
– Key Flex
Jan 7 at 17:03
2
$begingroup$
Okay thanks i think i understand now!
$endgroup$
– Viktor
Jan 7 at 17:03
add a comment |
$begingroup$
You can use Integral test:
$$f(n)=dfrac{n-1}{nsqrt{n+1}}$$
When you plot a graph, you will notice that $f(n)$ is positive, continuous and decreasing from $n=4$
So, $$int_4^infty dfrac{n-1}{nsqrt{n+1}}=mbox{diverges}$$
So, by integral test, the series diverges
$endgroup$
You can use Integral test:
$$f(n)=dfrac{n-1}{nsqrt{n+1}}$$
When you plot a graph, you will notice that $f(n)$ is positive, continuous and decreasing from $n=4$
So, $$int_4^infty dfrac{n-1}{nsqrt{n+1}}=mbox{diverges}$$
So, by integral test, the series diverges
answered Jan 7 at 16:56
Key FlexKey Flex
7,78461232
7,78461232
$begingroup$
I have 2 questions about this: first, why did you take out the negative sign? Can you take out the negative sign and when you do this and the remaining part converges or diverges, does it still converge or diverges, and second why can you start at 4 and not at 1 in your integral?
$endgroup$
– Viktor
Jan 7 at 16:59
1
$begingroup$
@Viktor first, I took the constant out, here $(-1)$, and then performed the test. This doesn't change the result. I took the boundary from $4$ only after checking if $f(n)$ is positive, continuous and decreasing.
$endgroup$
– Key Flex
Jan 7 at 17:03
2
$begingroup$
Okay thanks i think i understand now!
$endgroup$
– Viktor
Jan 7 at 17:03
add a comment |
$begingroup$
I have 2 questions about this: first, why did you take out the negative sign? Can you take out the negative sign and when you do this and the remaining part converges or diverges, does it still converge or diverges, and second why can you start at 4 and not at 1 in your integral?
$endgroup$
– Viktor
Jan 7 at 16:59
1
$begingroup$
@Viktor first, I took the constant out, here $(-1)$, and then performed the test. This doesn't change the result. I took the boundary from $4$ only after checking if $f(n)$ is positive, continuous and decreasing.
$endgroup$
– Key Flex
Jan 7 at 17:03
2
$begingroup$
Okay thanks i think i understand now!
$endgroup$
– Viktor
Jan 7 at 17:03
$begingroup$
I have 2 questions about this: first, why did you take out the negative sign? Can you take out the negative sign and when you do this and the remaining part converges or diverges, does it still converge or diverges, and second why can you start at 4 and not at 1 in your integral?
$endgroup$
– Viktor
Jan 7 at 16:59
$begingroup$
I have 2 questions about this: first, why did you take out the negative sign? Can you take out the negative sign and when you do this and the remaining part converges or diverges, does it still converge or diverges, and second why can you start at 4 and not at 1 in your integral?
$endgroup$
– Viktor
Jan 7 at 16:59
1
1
$begingroup$
@Viktor first, I took the constant out, here $(-1)$, and then performed the test. This doesn't change the result. I took the boundary from $4$ only after checking if $f(n)$ is positive, continuous and decreasing.
$endgroup$
– Key Flex
Jan 7 at 17:03
$begingroup$
@Viktor first, I took the constant out, here $(-1)$, and then performed the test. This doesn't change the result. I took the boundary from $4$ only after checking if $f(n)$ is positive, continuous and decreasing.
$endgroup$
– Key Flex
Jan 7 at 17:03
2
2
$begingroup$
Okay thanks i think i understand now!
$endgroup$
– Viktor
Jan 7 at 17:03
$begingroup$
Okay thanks i think i understand now!
$endgroup$
– Viktor
Jan 7 at 17:03
add a comment |
$begingroup$
For $n>2$,
$$frac{n-1}{nsqrt{n+1}}>frac1{2sqrt{n+1}} $$
As $sum frac1{2sqrt{n+1}}$ diverges. so does $sumfrac{n-1}{nsqrt{n+1}}$ and also $sumfrac{-(n-1)}{nsqrt{n+1}}$.
$endgroup$
add a comment |
$begingroup$
For $n>2$,
$$frac{n-1}{nsqrt{n+1}}>frac1{2sqrt{n+1}} $$
As $sum frac1{2sqrt{n+1}}$ diverges. so does $sumfrac{n-1}{nsqrt{n+1}}$ and also $sumfrac{-(n-1)}{nsqrt{n+1}}$.
$endgroup$
add a comment |
$begingroup$
For $n>2$,
$$frac{n-1}{nsqrt{n+1}}>frac1{2sqrt{n+1}} $$
As $sum frac1{2sqrt{n+1}}$ diverges. so does $sumfrac{n-1}{nsqrt{n+1}}$ and also $sumfrac{-(n-1)}{nsqrt{n+1}}$.
$endgroup$
For $n>2$,
$$frac{n-1}{nsqrt{n+1}}>frac1{2sqrt{n+1}} $$
As $sum frac1{2sqrt{n+1}}$ diverges. so does $sumfrac{n-1}{nsqrt{n+1}}$ and also $sumfrac{-(n-1)}{nsqrt{n+1}}$.
answered Jan 7 at 16:58
Hagen von EitzenHagen von Eitzen
277k22269496
277k22269496
add a comment |
add a comment |
$begingroup$
You may also use the comparison test.
Intuitively, as $n$ gets larger, $n-1$ is essentially $n$, and $sqrt{n+1}$ is essentially $sqrt{n}$. That means you can compare $frac{n-1}{nsqrt{n+1}}$ with $frac{n}{nsqrt{n}}=frac{1}{sqrt{n}}$
Formally, you can find a constant $C>0$ such that:
$$frac{n-1}{nsqrt{n+1}}>Cfrac{1}{sqrt{n}}, text{ for }ngeq 2$$
(what could that constant be?)
Now since $sum_{n=1}^inftyfrac{1}{sqrt{n}}$ diverges, that means $sum_{n=1}^inftyfrac{n-1}{nsqrt{n+1}}$ diverges.
Obs.: For any non-zero constant $lambda$, we have that $sum_{n=1}^infty lambda, a_n$ converges $Leftrightarrow sum_{n=1}^infty a_n$ converges (this should be a simple exercise). In particular, the negative sign in $frac{-(n-1)}{nsqrt{n+1}}$ is not relevant to convergence).
$endgroup$
add a comment |
$begingroup$
You may also use the comparison test.
Intuitively, as $n$ gets larger, $n-1$ is essentially $n$, and $sqrt{n+1}$ is essentially $sqrt{n}$. That means you can compare $frac{n-1}{nsqrt{n+1}}$ with $frac{n}{nsqrt{n}}=frac{1}{sqrt{n}}$
Formally, you can find a constant $C>0$ such that:
$$frac{n-1}{nsqrt{n+1}}>Cfrac{1}{sqrt{n}}, text{ for }ngeq 2$$
(what could that constant be?)
Now since $sum_{n=1}^inftyfrac{1}{sqrt{n}}$ diverges, that means $sum_{n=1}^inftyfrac{n-1}{nsqrt{n+1}}$ diverges.
Obs.: For any non-zero constant $lambda$, we have that $sum_{n=1}^infty lambda, a_n$ converges $Leftrightarrow sum_{n=1}^infty a_n$ converges (this should be a simple exercise). In particular, the negative sign in $frac{-(n-1)}{nsqrt{n+1}}$ is not relevant to convergence).
$endgroup$
add a comment |
$begingroup$
You may also use the comparison test.
Intuitively, as $n$ gets larger, $n-1$ is essentially $n$, and $sqrt{n+1}$ is essentially $sqrt{n}$. That means you can compare $frac{n-1}{nsqrt{n+1}}$ with $frac{n}{nsqrt{n}}=frac{1}{sqrt{n}}$
Formally, you can find a constant $C>0$ such that:
$$frac{n-1}{nsqrt{n+1}}>Cfrac{1}{sqrt{n}}, text{ for }ngeq 2$$
(what could that constant be?)
Now since $sum_{n=1}^inftyfrac{1}{sqrt{n}}$ diverges, that means $sum_{n=1}^inftyfrac{n-1}{nsqrt{n+1}}$ diverges.
Obs.: For any non-zero constant $lambda$, we have that $sum_{n=1}^infty lambda, a_n$ converges $Leftrightarrow sum_{n=1}^infty a_n$ converges (this should be a simple exercise). In particular, the negative sign in $frac{-(n-1)}{nsqrt{n+1}}$ is not relevant to convergence).
$endgroup$
You may also use the comparison test.
Intuitively, as $n$ gets larger, $n-1$ is essentially $n$, and $sqrt{n+1}$ is essentially $sqrt{n}$. That means you can compare $frac{n-1}{nsqrt{n+1}}$ with $frac{n}{nsqrt{n}}=frac{1}{sqrt{n}}$
Formally, you can find a constant $C>0$ such that:
$$frac{n-1}{nsqrt{n+1}}>Cfrac{1}{sqrt{n}}, text{ for }ngeq 2$$
(what could that constant be?)
Now since $sum_{n=1}^inftyfrac{1}{sqrt{n}}$ diverges, that means $sum_{n=1}^inftyfrac{n-1}{nsqrt{n+1}}$ diverges.
Obs.: For any non-zero constant $lambda$, we have that $sum_{n=1}^infty lambda, a_n$ converges $Leftrightarrow sum_{n=1}^infty a_n$ converges (this should be a simple exercise). In particular, the negative sign in $frac{-(n-1)}{nsqrt{n+1}}$ is not relevant to convergence).
edited Jan 15 at 14:56
answered Jan 7 at 17:12
rmdmc89rmdmc89
2,1071922
2,1071922
add a comment |
add a comment |
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$begingroup$
then get rid of the $-1$, since $sum frac{1}{n^{3/2}}$ converges. so you have $sum_n frac{1}{sqrt{n}}$, which diverges
$endgroup$
– mathworker21
Jan 7 at 16:54
$begingroup$
@CalvinGodfrey I don't think that's "without loss of generality"
$endgroup$
– mathworker21
Jan 7 at 16:55