Evaluate $sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)}$
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Evaluate
$$sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)}$$
So in a previous part of the question I calculated that
$$sum_{r=1}^{n} frac{2-r}{r(r+1)(r+2)} = sum_{r=1}^{n}left( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=frac{n}{(n+1)(n+2)}$$
So my question is then why does $$sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)} = -frac{1}{6}$$
As I though that it would be $frac{1}{2}$.
summation
$endgroup$
add a comment |
$begingroup$
Evaluate
$$sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)}$$
So in a previous part of the question I calculated that
$$sum_{r=1}^{n} frac{2-r}{r(r+1)(r+2)} = sum_{r=1}^{n}left( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=frac{n}{(n+1)(n+2)}$$
So my question is then why does $$sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)} = -frac{1}{6}$$
As I though that it would be $frac{1}{2}$.
summation
$endgroup$
add a comment |
$begingroup$
Evaluate
$$sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)}$$
So in a previous part of the question I calculated that
$$sum_{r=1}^{n} frac{2-r}{r(r+1)(r+2)} = sum_{r=1}^{n}left( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=frac{n}{(n+1)(n+2)}$$
So my question is then why does $$sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)} = -frac{1}{6}$$
As I though that it would be $frac{1}{2}$.
summation
$endgroup$
Evaluate
$$sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)}$$
So in a previous part of the question I calculated that
$$sum_{r=1}^{n} frac{2-r}{r(r+1)(r+2)} = sum_{r=1}^{n}left( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=frac{n}{(n+1)(n+2)}$$
So my question is then why does $$sum_{r=2}^{infty} frac{2-r}{r(r+1)(r+2)} = -frac{1}{6}$$
As I though that it would be $frac{1}{2}$.
summation
summation
asked Dec 31 '18 at 15:47
H.LinkhornH.Linkhorn
537313
537313
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6 Answers
6
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Note thatbegin{align}frac{2-r}{r(r+1)(r+2)}&=frac1r-frac3{r+1}+frac2{r+2}\&=frac1r-frac1{r+1}-frac2{r+1}+frac2{r+2}.end{align}Therefore, your series is naturally the sum of two telescoping series and its sum is$$frac12-frac23=-frac16.$$
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$begingroup$
(+1) was a minute late!
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– TheSimpliFire
Dec 31 '18 at 15:54
add a comment |
$begingroup$
Note that begin{align}sum_{r=2}^inftyleft( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)&=sum_{r=2}^inftyleft(frac1r-frac1{r+1}right)-2sum_{r=2}^inftyleft(frac1{r+1}-frac1{r+2}right)\&=left(frac12-frac13+frac13-frac14+cdotsright)-2left(frac13+frac14-frac14+frac15-cdotsright)\&=frac12-frac23=-frac16end{align}
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1
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(+1) Believe me: I know what it feels to be a few seconds too late.
$endgroup$
– José Carlos Santos
Dec 31 '18 at 15:55
add a comment |
$begingroup$
Actually your finite sum is equal to $$sum_{r=2}^nfrac{2-r}{r(r+1)(r+2)}=frac{2}{n+2}-frac{1}{n+1}-frac{1}{6}$$
$endgroup$
add a comment |
$begingroup$
Let $dfrac{2-r}{r(r+1)(r+2)}=f(r)-f(r+1),$ where$ f(m)=dfrac{am+b}{m(m+1)}$
$implies2-r=(r+2)(ar+b)-r(ar+a+b)=ar+2b$
Set $2b=2iff b=1,ar=-riff a=-1$
$$impliessum_{r=1}^ndfrac{2-r}{r(r+1)(r+2)}=sum_{r=1}^nleft(f(r)-f(r+1)right)=f(1)-f(n+1)$$
Set $ntoinfty$ and show that $ntoinfty f(n)=0$
$endgroup$
add a comment |
$begingroup$
If $H_{n}$ is the $n$-th harmonic number, we have $$sum_{r=2}^{n}left(frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=left(H_{n}-1right)-3left(H_{n}-1-frac{1}{2}right)+2left(H_{n}-1-frac{1}{2}-frac{1}{3}right)$$ $$=-1+frac{3}{2}-frac{2}{3}=-frac{1}{6}.$$ Now take the limit as $nrightarrow+infty$.
$endgroup$
add a comment |
$begingroup$
By definition,
$$ sum_{r=1}^infty a_r = lim_{ntoinfty}sum_{r=1}^n a_r .$$
In your case the above limit is zero. Then:
$$ sum_{r=2}^infty a_r = sum_{r=1}^infty a_r - a_1 = -a_1 . $$
This reasoning is what @HenriLee was referring to in his answer.
$endgroup$
add a comment |
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6 Answers
6
active
oldest
votes
6 Answers
6
active
oldest
votes
active
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votes
$begingroup$
Note thatbegin{align}frac{2-r}{r(r+1)(r+2)}&=frac1r-frac3{r+1}+frac2{r+2}\&=frac1r-frac1{r+1}-frac2{r+1}+frac2{r+2}.end{align}Therefore, your series is naturally the sum of two telescoping series and its sum is$$frac12-frac23=-frac16.$$
$endgroup$
$begingroup$
(+1) was a minute late!
$endgroup$
– TheSimpliFire
Dec 31 '18 at 15:54
add a comment |
$begingroup$
Note thatbegin{align}frac{2-r}{r(r+1)(r+2)}&=frac1r-frac3{r+1}+frac2{r+2}\&=frac1r-frac1{r+1}-frac2{r+1}+frac2{r+2}.end{align}Therefore, your series is naturally the sum of two telescoping series and its sum is$$frac12-frac23=-frac16.$$
$endgroup$
$begingroup$
(+1) was a minute late!
$endgroup$
– TheSimpliFire
Dec 31 '18 at 15:54
add a comment |
$begingroup$
Note thatbegin{align}frac{2-r}{r(r+1)(r+2)}&=frac1r-frac3{r+1}+frac2{r+2}\&=frac1r-frac1{r+1}-frac2{r+1}+frac2{r+2}.end{align}Therefore, your series is naturally the sum of two telescoping series and its sum is$$frac12-frac23=-frac16.$$
$endgroup$
Note thatbegin{align}frac{2-r}{r(r+1)(r+2)}&=frac1r-frac3{r+1}+frac2{r+2}\&=frac1r-frac1{r+1}-frac2{r+1}+frac2{r+2}.end{align}Therefore, your series is naturally the sum of two telescoping series and its sum is$$frac12-frac23=-frac16.$$
answered Dec 31 '18 at 15:52
José Carlos SantosJosé Carlos Santos
178k24139251
178k24139251
$begingroup$
(+1) was a minute late!
$endgroup$
– TheSimpliFire
Dec 31 '18 at 15:54
add a comment |
$begingroup$
(+1) was a minute late!
$endgroup$
– TheSimpliFire
Dec 31 '18 at 15:54
$begingroup$
(+1) was a minute late!
$endgroup$
– TheSimpliFire
Dec 31 '18 at 15:54
$begingroup$
(+1) was a minute late!
$endgroup$
– TheSimpliFire
Dec 31 '18 at 15:54
add a comment |
$begingroup$
Note that begin{align}sum_{r=2}^inftyleft( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)&=sum_{r=2}^inftyleft(frac1r-frac1{r+1}right)-2sum_{r=2}^inftyleft(frac1{r+1}-frac1{r+2}right)\&=left(frac12-frac13+frac13-frac14+cdotsright)-2left(frac13+frac14-frac14+frac15-cdotsright)\&=frac12-frac23=-frac16end{align}
$endgroup$
1
$begingroup$
(+1) Believe me: I know what it feels to be a few seconds too late.
$endgroup$
– José Carlos Santos
Dec 31 '18 at 15:55
add a comment |
$begingroup$
Note that begin{align}sum_{r=2}^inftyleft( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)&=sum_{r=2}^inftyleft(frac1r-frac1{r+1}right)-2sum_{r=2}^inftyleft(frac1{r+1}-frac1{r+2}right)\&=left(frac12-frac13+frac13-frac14+cdotsright)-2left(frac13+frac14-frac14+frac15-cdotsright)\&=frac12-frac23=-frac16end{align}
$endgroup$
1
$begingroup$
(+1) Believe me: I know what it feels to be a few seconds too late.
$endgroup$
– José Carlos Santos
Dec 31 '18 at 15:55
add a comment |
$begingroup$
Note that begin{align}sum_{r=2}^inftyleft( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)&=sum_{r=2}^inftyleft(frac1r-frac1{r+1}right)-2sum_{r=2}^inftyleft(frac1{r+1}-frac1{r+2}right)\&=left(frac12-frac13+frac13-frac14+cdotsright)-2left(frac13+frac14-frac14+frac15-cdotsright)\&=frac12-frac23=-frac16end{align}
$endgroup$
Note that begin{align}sum_{r=2}^inftyleft( frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)&=sum_{r=2}^inftyleft(frac1r-frac1{r+1}right)-2sum_{r=2}^inftyleft(frac1{r+1}-frac1{r+2}right)\&=left(frac12-frac13+frac13-frac14+cdotsright)-2left(frac13+frac14-frac14+frac15-cdotsright)\&=frac12-frac23=-frac16end{align}
answered Dec 31 '18 at 15:53
TheSimpliFireTheSimpliFire
13.6k62765
13.6k62765
1
$begingroup$
(+1) Believe me: I know what it feels to be a few seconds too late.
$endgroup$
– José Carlos Santos
Dec 31 '18 at 15:55
add a comment |
1
$begingroup$
(+1) Believe me: I know what it feels to be a few seconds too late.
$endgroup$
– José Carlos Santos
Dec 31 '18 at 15:55
1
1
$begingroup$
(+1) Believe me: I know what it feels to be a few seconds too late.
$endgroup$
– José Carlos Santos
Dec 31 '18 at 15:55
$begingroup$
(+1) Believe me: I know what it feels to be a few seconds too late.
$endgroup$
– José Carlos Santos
Dec 31 '18 at 15:55
add a comment |
$begingroup$
Actually your finite sum is equal to $$sum_{r=2}^nfrac{2-r}{r(r+1)(r+2)}=frac{2}{n+2}-frac{1}{n+1}-frac{1}{6}$$
$endgroup$
add a comment |
$begingroup$
Actually your finite sum is equal to $$sum_{r=2}^nfrac{2-r}{r(r+1)(r+2)}=frac{2}{n+2}-frac{1}{n+1}-frac{1}{6}$$
$endgroup$
add a comment |
$begingroup$
Actually your finite sum is equal to $$sum_{r=2}^nfrac{2-r}{r(r+1)(r+2)}=frac{2}{n+2}-frac{1}{n+1}-frac{1}{6}$$
$endgroup$
Actually your finite sum is equal to $$sum_{r=2}^nfrac{2-r}{r(r+1)(r+2)}=frac{2}{n+2}-frac{1}{n+1}-frac{1}{6}$$
answered Dec 31 '18 at 15:54
Dr. Sonnhard GraubnerDr. Sonnhard Graubner
79.9k42867
79.9k42867
add a comment |
add a comment |
$begingroup$
Let $dfrac{2-r}{r(r+1)(r+2)}=f(r)-f(r+1),$ where$ f(m)=dfrac{am+b}{m(m+1)}$
$implies2-r=(r+2)(ar+b)-r(ar+a+b)=ar+2b$
Set $2b=2iff b=1,ar=-riff a=-1$
$$impliessum_{r=1}^ndfrac{2-r}{r(r+1)(r+2)}=sum_{r=1}^nleft(f(r)-f(r+1)right)=f(1)-f(n+1)$$
Set $ntoinfty$ and show that $ntoinfty f(n)=0$
$endgroup$
add a comment |
$begingroup$
Let $dfrac{2-r}{r(r+1)(r+2)}=f(r)-f(r+1),$ where$ f(m)=dfrac{am+b}{m(m+1)}$
$implies2-r=(r+2)(ar+b)-r(ar+a+b)=ar+2b$
Set $2b=2iff b=1,ar=-riff a=-1$
$$impliessum_{r=1}^ndfrac{2-r}{r(r+1)(r+2)}=sum_{r=1}^nleft(f(r)-f(r+1)right)=f(1)-f(n+1)$$
Set $ntoinfty$ and show that $ntoinfty f(n)=0$
$endgroup$
add a comment |
$begingroup$
Let $dfrac{2-r}{r(r+1)(r+2)}=f(r)-f(r+1),$ where$ f(m)=dfrac{am+b}{m(m+1)}$
$implies2-r=(r+2)(ar+b)-r(ar+a+b)=ar+2b$
Set $2b=2iff b=1,ar=-riff a=-1$
$$impliessum_{r=1}^ndfrac{2-r}{r(r+1)(r+2)}=sum_{r=1}^nleft(f(r)-f(r+1)right)=f(1)-f(n+1)$$
Set $ntoinfty$ and show that $ntoinfty f(n)=0$
$endgroup$
Let $dfrac{2-r}{r(r+1)(r+2)}=f(r)-f(r+1),$ where$ f(m)=dfrac{am+b}{m(m+1)}$
$implies2-r=(r+2)(ar+b)-r(ar+a+b)=ar+2b$
Set $2b=2iff b=1,ar=-riff a=-1$
$$impliessum_{r=1}^ndfrac{2-r}{r(r+1)(r+2)}=sum_{r=1}^nleft(f(r)-f(r+1)right)=f(1)-f(n+1)$$
Set $ntoinfty$ and show that $ntoinfty f(n)=0$
answered Dec 31 '18 at 16:07
lab bhattacharjeelab bhattacharjee
229k15159280
229k15159280
add a comment |
add a comment |
$begingroup$
If $H_{n}$ is the $n$-th harmonic number, we have $$sum_{r=2}^{n}left(frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=left(H_{n}-1right)-3left(H_{n}-1-frac{1}{2}right)+2left(H_{n}-1-frac{1}{2}-frac{1}{3}right)$$ $$=-1+frac{3}{2}-frac{2}{3}=-frac{1}{6}.$$ Now take the limit as $nrightarrow+infty$.
$endgroup$
add a comment |
$begingroup$
If $H_{n}$ is the $n$-th harmonic number, we have $$sum_{r=2}^{n}left(frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=left(H_{n}-1right)-3left(H_{n}-1-frac{1}{2}right)+2left(H_{n}-1-frac{1}{2}-frac{1}{3}right)$$ $$=-1+frac{3}{2}-frac{2}{3}=-frac{1}{6}.$$ Now take the limit as $nrightarrow+infty$.
$endgroup$
add a comment |
$begingroup$
If $H_{n}$ is the $n$-th harmonic number, we have $$sum_{r=2}^{n}left(frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=left(H_{n}-1right)-3left(H_{n}-1-frac{1}{2}right)+2left(H_{n}-1-frac{1}{2}-frac{1}{3}right)$$ $$=-1+frac{3}{2}-frac{2}{3}=-frac{1}{6}.$$ Now take the limit as $nrightarrow+infty$.
$endgroup$
If $H_{n}$ is the $n$-th harmonic number, we have $$sum_{r=2}^{n}left(frac{1}{r}-frac{3}{r+1}+frac{2}{r+2}right)=left(H_{n}-1right)-3left(H_{n}-1-frac{1}{2}right)+2left(H_{n}-1-frac{1}{2}-frac{1}{3}right)$$ $$=-1+frac{3}{2}-frac{2}{3}=-frac{1}{6}.$$ Now take the limit as $nrightarrow+infty$.
answered Dec 31 '18 at 16:08
Marco CantariniMarco Cantarini
28.8k23373
28.8k23373
add a comment |
add a comment |
$begingroup$
By definition,
$$ sum_{r=1}^infty a_r = lim_{ntoinfty}sum_{r=1}^n a_r .$$
In your case the above limit is zero. Then:
$$ sum_{r=2}^infty a_r = sum_{r=1}^infty a_r - a_1 = -a_1 . $$
This reasoning is what @HenriLee was referring to in his answer.
$endgroup$
add a comment |
$begingroup$
By definition,
$$ sum_{r=1}^infty a_r = lim_{ntoinfty}sum_{r=1}^n a_r .$$
In your case the above limit is zero. Then:
$$ sum_{r=2}^infty a_r = sum_{r=1}^infty a_r - a_1 = -a_1 . $$
This reasoning is what @HenriLee was referring to in his answer.
$endgroup$
add a comment |
$begingroup$
By definition,
$$ sum_{r=1}^infty a_r = lim_{ntoinfty}sum_{r=1}^n a_r .$$
In your case the above limit is zero. Then:
$$ sum_{r=2}^infty a_r = sum_{r=1}^infty a_r - a_1 = -a_1 . $$
This reasoning is what @HenriLee was referring to in his answer.
$endgroup$
By definition,
$$ sum_{r=1}^infty a_r = lim_{ntoinfty}sum_{r=1}^n a_r .$$
In your case the above limit is zero. Then:
$$ sum_{r=2}^infty a_r = sum_{r=1}^infty a_r - a_1 = -a_1 . $$
This reasoning is what @HenriLee was referring to in his answer.
answered Dec 31 '18 at 15:57
Dog_69Dog_69
6361523
6361523
add a comment |
add a comment |
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