Prove $sum_{r=2}^n {n choose r} r(r-1) = n(n-1)2^{n-2}$ for $ngeq 2$
$begingroup$
How to prove following:
$$sum_{r=2}^n {n choose r} r(r-1) = n(n-1)2^{n-2}$$ for $ngeq 2$
Thanks!!
discrete-mathematics proof-explanation
$endgroup$
add a comment |
$begingroup$
How to prove following:
$$sum_{r=2}^n {n choose r} r(r-1) = n(n-1)2^{n-2}$$ for $ngeq 2$
Thanks!!
discrete-mathematics proof-explanation
$endgroup$
1
$begingroup$
I'd use induction. Start with $n = 2$ and then prove that if the statement is valid for $n$ it is also valid for $n+1$. Basically, people get more enthusiastic about questions if the poster shows hir/her efforts
$endgroup$
– Ronald
Dec 6 '18 at 10:17
add a comment |
$begingroup$
How to prove following:
$$sum_{r=2}^n {n choose r} r(r-1) = n(n-1)2^{n-2}$$ for $ngeq 2$
Thanks!!
discrete-mathematics proof-explanation
$endgroup$
How to prove following:
$$sum_{r=2}^n {n choose r} r(r-1) = n(n-1)2^{n-2}$$ for $ngeq 2$
Thanks!!
discrete-mathematics proof-explanation
discrete-mathematics proof-explanation
edited Dec 6 '18 at 10:21
greedoid
41.3k1150102
41.3k1150102
asked Dec 6 '18 at 10:15
TimgascdTimgascd
303
303
1
$begingroup$
I'd use induction. Start with $n = 2$ and then prove that if the statement is valid for $n$ it is also valid for $n+1$. Basically, people get more enthusiastic about questions if the poster shows hir/her efforts
$endgroup$
– Ronald
Dec 6 '18 at 10:17
add a comment |
1
$begingroup$
I'd use induction. Start with $n = 2$ and then prove that if the statement is valid for $n$ it is also valid for $n+1$. Basically, people get more enthusiastic about questions if the poster shows hir/her efforts
$endgroup$
– Ronald
Dec 6 '18 at 10:17
1
1
$begingroup$
I'd use induction. Start with $n = 2$ and then prove that if the statement is valid for $n$ it is also valid for $n+1$. Basically, people get more enthusiastic about questions if the poster shows hir/her efforts
$endgroup$
– Ronald
Dec 6 '18 at 10:17
$begingroup$
I'd use induction. Start with $n = 2$ and then prove that if the statement is valid for $n$ it is also valid for $n+1$. Basically, people get more enthusiastic about questions if the poster shows hir/her efforts
$endgroup$
– Ronald
Dec 6 '18 at 10:17
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
Hint:
For $r(r-1)ne0$
$$r(r-1)binom nr=r(r-1)n(n-1)dfrac{(n-2)!}{r(r-1)cdot(r-2)!cdot{n-2-(r-2)}!}=n(n-1)binom{n-2}{r-2}$$
Now in
$$(a+b)^m=sum_{r=0}^mbinom mr a^{m-r}b^r$$
put $a=b=1, m=n-2$
Some observations :
$$sum_{r=2}^nbinom nrr(r-1)=sum_{r=0}^nbinom nrr(r-1)$$
The proposition trivially holds true for $n=0,1$
$endgroup$
$begingroup$
Sorry, I do not really understand all of them. Can you explain it in detail? Thanks
$endgroup$
– Timgascd
Dec 6 '18 at 10:41
$begingroup$
@Hgascd, All of them or none of them? $$binom nr=dfrac{n!}{(n-r)! r!}=dfrac{n(n-1)}{r(r-1)}cdotdfrac{(n-2)!}{(r-2)!cdot{n-2-(r-2)}!}=$$
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 10:55
$begingroup$
Thanks a lot. I got it
$endgroup$
– Timgascd
Dec 6 '18 at 11:04
add a comment |
$begingroup$
Question: On how many ways can we choose a group in a set of $n$ people and then president and then vicepresident?
Well we can first a group of $r$ people, that is ${nchoose r}$, for every $rleq n$, and then president among them, so we have $r$ choises and then $r-1$ choises for V.P. Suming (we can sum from $0$) for all $r$ we get:
$$sum_{r=0}^{n} r(r-1)C_{n}^{r} $$
On the other hand we can first choose a president among all people, so we have $n$ posibilities and then V.P. for who we have $n-1$ choises and then we choose any set in set of $n-2$ people, for that we have $2^{n-2}$ choises, so:
$$n(n-1)2^{n-2}$$ and this is the answer to your question.
$endgroup$
add a comment |
$begingroup$
Hint: Take $f(x)=(1+x)^n$ and consider $f''(1)$.
$endgroup$
add a comment |
Your Answer
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
Hint:
For $r(r-1)ne0$
$$r(r-1)binom nr=r(r-1)n(n-1)dfrac{(n-2)!}{r(r-1)cdot(r-2)!cdot{n-2-(r-2)}!}=n(n-1)binom{n-2}{r-2}$$
Now in
$$(a+b)^m=sum_{r=0}^mbinom mr a^{m-r}b^r$$
put $a=b=1, m=n-2$
Some observations :
$$sum_{r=2}^nbinom nrr(r-1)=sum_{r=0}^nbinom nrr(r-1)$$
The proposition trivially holds true for $n=0,1$
$endgroup$
$begingroup$
Sorry, I do not really understand all of them. Can you explain it in detail? Thanks
$endgroup$
– Timgascd
Dec 6 '18 at 10:41
$begingroup$
@Hgascd, All of them or none of them? $$binom nr=dfrac{n!}{(n-r)! r!}=dfrac{n(n-1)}{r(r-1)}cdotdfrac{(n-2)!}{(r-2)!cdot{n-2-(r-2)}!}=$$
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 10:55
$begingroup$
Thanks a lot. I got it
$endgroup$
– Timgascd
Dec 6 '18 at 11:04
add a comment |
$begingroup$
Hint:
For $r(r-1)ne0$
$$r(r-1)binom nr=r(r-1)n(n-1)dfrac{(n-2)!}{r(r-1)cdot(r-2)!cdot{n-2-(r-2)}!}=n(n-1)binom{n-2}{r-2}$$
Now in
$$(a+b)^m=sum_{r=0}^mbinom mr a^{m-r}b^r$$
put $a=b=1, m=n-2$
Some observations :
$$sum_{r=2}^nbinom nrr(r-1)=sum_{r=0}^nbinom nrr(r-1)$$
The proposition trivially holds true for $n=0,1$
$endgroup$
$begingroup$
Sorry, I do not really understand all of them. Can you explain it in detail? Thanks
$endgroup$
– Timgascd
Dec 6 '18 at 10:41
$begingroup$
@Hgascd, All of them or none of them? $$binom nr=dfrac{n!}{(n-r)! r!}=dfrac{n(n-1)}{r(r-1)}cdotdfrac{(n-2)!}{(r-2)!cdot{n-2-(r-2)}!}=$$
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 10:55
$begingroup$
Thanks a lot. I got it
$endgroup$
– Timgascd
Dec 6 '18 at 11:04
add a comment |
$begingroup$
Hint:
For $r(r-1)ne0$
$$r(r-1)binom nr=r(r-1)n(n-1)dfrac{(n-2)!}{r(r-1)cdot(r-2)!cdot{n-2-(r-2)}!}=n(n-1)binom{n-2}{r-2}$$
Now in
$$(a+b)^m=sum_{r=0}^mbinom mr a^{m-r}b^r$$
put $a=b=1, m=n-2$
Some observations :
$$sum_{r=2}^nbinom nrr(r-1)=sum_{r=0}^nbinom nrr(r-1)$$
The proposition trivially holds true for $n=0,1$
$endgroup$
Hint:
For $r(r-1)ne0$
$$r(r-1)binom nr=r(r-1)n(n-1)dfrac{(n-2)!}{r(r-1)cdot(r-2)!cdot{n-2-(r-2)}!}=n(n-1)binom{n-2}{r-2}$$
Now in
$$(a+b)^m=sum_{r=0}^mbinom mr a^{m-r}b^r$$
put $a=b=1, m=n-2$
Some observations :
$$sum_{r=2}^nbinom nrr(r-1)=sum_{r=0}^nbinom nrr(r-1)$$
The proposition trivially holds true for $n=0,1$
answered Dec 6 '18 at 10:20
lab bhattacharjeelab bhattacharjee
225k15157275
225k15157275
$begingroup$
Sorry, I do not really understand all of them. Can you explain it in detail? Thanks
$endgroup$
– Timgascd
Dec 6 '18 at 10:41
$begingroup$
@Hgascd, All of them or none of them? $$binom nr=dfrac{n!}{(n-r)! r!}=dfrac{n(n-1)}{r(r-1)}cdotdfrac{(n-2)!}{(r-2)!cdot{n-2-(r-2)}!}=$$
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 10:55
$begingroup$
Thanks a lot. I got it
$endgroup$
– Timgascd
Dec 6 '18 at 11:04
add a comment |
$begingroup$
Sorry, I do not really understand all of them. Can you explain it in detail? Thanks
$endgroup$
– Timgascd
Dec 6 '18 at 10:41
$begingroup$
@Hgascd, All of them or none of them? $$binom nr=dfrac{n!}{(n-r)! r!}=dfrac{n(n-1)}{r(r-1)}cdotdfrac{(n-2)!}{(r-2)!cdot{n-2-(r-2)}!}=$$
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 10:55
$begingroup$
Thanks a lot. I got it
$endgroup$
– Timgascd
Dec 6 '18 at 11:04
$begingroup$
Sorry, I do not really understand all of them. Can you explain it in detail? Thanks
$endgroup$
– Timgascd
Dec 6 '18 at 10:41
$begingroup$
Sorry, I do not really understand all of them. Can you explain it in detail? Thanks
$endgroup$
– Timgascd
Dec 6 '18 at 10:41
$begingroup$
@Hgascd, All of them or none of them? $$binom nr=dfrac{n!}{(n-r)! r!}=dfrac{n(n-1)}{r(r-1)}cdotdfrac{(n-2)!}{(r-2)!cdot{n-2-(r-2)}!}=$$
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 10:55
$begingroup$
@Hgascd, All of them or none of them? $$binom nr=dfrac{n!}{(n-r)! r!}=dfrac{n(n-1)}{r(r-1)}cdotdfrac{(n-2)!}{(r-2)!cdot{n-2-(r-2)}!}=$$
$endgroup$
– lab bhattacharjee
Dec 6 '18 at 10:55
$begingroup$
Thanks a lot. I got it
$endgroup$
– Timgascd
Dec 6 '18 at 11:04
$begingroup$
Thanks a lot. I got it
$endgroup$
– Timgascd
Dec 6 '18 at 11:04
add a comment |
$begingroup$
Question: On how many ways can we choose a group in a set of $n$ people and then president and then vicepresident?
Well we can first a group of $r$ people, that is ${nchoose r}$, for every $rleq n$, and then president among them, so we have $r$ choises and then $r-1$ choises for V.P. Suming (we can sum from $0$) for all $r$ we get:
$$sum_{r=0}^{n} r(r-1)C_{n}^{r} $$
On the other hand we can first choose a president among all people, so we have $n$ posibilities and then V.P. for who we have $n-1$ choises and then we choose any set in set of $n-2$ people, for that we have $2^{n-2}$ choises, so:
$$n(n-1)2^{n-2}$$ and this is the answer to your question.
$endgroup$
add a comment |
$begingroup$
Question: On how many ways can we choose a group in a set of $n$ people and then president and then vicepresident?
Well we can first a group of $r$ people, that is ${nchoose r}$, for every $rleq n$, and then president among them, so we have $r$ choises and then $r-1$ choises for V.P. Suming (we can sum from $0$) for all $r$ we get:
$$sum_{r=0}^{n} r(r-1)C_{n}^{r} $$
On the other hand we can first choose a president among all people, so we have $n$ posibilities and then V.P. for who we have $n-1$ choises and then we choose any set in set of $n-2$ people, for that we have $2^{n-2}$ choises, so:
$$n(n-1)2^{n-2}$$ and this is the answer to your question.
$endgroup$
add a comment |
$begingroup$
Question: On how many ways can we choose a group in a set of $n$ people and then president and then vicepresident?
Well we can first a group of $r$ people, that is ${nchoose r}$, for every $rleq n$, and then president among them, so we have $r$ choises and then $r-1$ choises for V.P. Suming (we can sum from $0$) for all $r$ we get:
$$sum_{r=0}^{n} r(r-1)C_{n}^{r} $$
On the other hand we can first choose a president among all people, so we have $n$ posibilities and then V.P. for who we have $n-1$ choises and then we choose any set in set of $n-2$ people, for that we have $2^{n-2}$ choises, so:
$$n(n-1)2^{n-2}$$ and this is the answer to your question.
$endgroup$
Question: On how many ways can we choose a group in a set of $n$ people and then president and then vicepresident?
Well we can first a group of $r$ people, that is ${nchoose r}$, for every $rleq n$, and then president among them, so we have $r$ choises and then $r-1$ choises for V.P. Suming (we can sum from $0$) for all $r$ we get:
$$sum_{r=0}^{n} r(r-1)C_{n}^{r} $$
On the other hand we can first choose a president among all people, so we have $n$ posibilities and then V.P. for who we have $n-1$ choises and then we choose any set in set of $n-2$ people, for that we have $2^{n-2}$ choises, so:
$$n(n-1)2^{n-2}$$ and this is the answer to your question.
answered Dec 6 '18 at 10:20
greedoidgreedoid
41.3k1150102
41.3k1150102
add a comment |
add a comment |
$begingroup$
Hint: Take $f(x)=(1+x)^n$ and consider $f''(1)$.
$endgroup$
add a comment |
$begingroup$
Hint: Take $f(x)=(1+x)^n$ and consider $f''(1)$.
$endgroup$
add a comment |
$begingroup$
Hint: Take $f(x)=(1+x)^n$ and consider $f''(1)$.
$endgroup$
Hint: Take $f(x)=(1+x)^n$ and consider $f''(1)$.
answered Dec 6 '18 at 10:20
lhflhf
165k10171395
165k10171395
add a comment |
add a comment |
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$begingroup$
I'd use induction. Start with $n = 2$ and then prove that if the statement is valid for $n$ it is also valid for $n+1$. Basically, people get more enthusiastic about questions if the poster shows hir/her efforts
$endgroup$
– Ronald
Dec 6 '18 at 10:17