equating coefficients in algebraic expansion
$begingroup$
If $displaystyle bigg(frac{1+x}{1-x}bigg)^n=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots +infty,$ then value of
$(1); displaystyle frac{3b_{3}-b_{1}}{b_{2}}$
$(2); displaystyle frac{2b_{4}-b_{2}}{b_{3}}$
$(3); displaystyle frac{3b_{6}-2b_{4}}{b_{5}}$
$(4); displaystyle frac{5b_{10}-4b_{8}}{b_{9}}$
Answers given in $n,2n,3n,4n$ formats
What I tried:
$displaystyle (1+x)^n(1-x)^{-n}=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots cdots infty$
$displaystyle bigg[1+nx+frac{n(n-1)}{2}x^2+frac{n(n-1)(n-2)}{6}cdots bigg]bigg[1+nx+frac{n(n+1)}{2}x^2+frac{n(n+1)(n+2)}{6}x^3+cdots bigg]=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots $
$displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$
How do I find other coefficients? Help me, please.
binomial-theorem
$endgroup$
add a comment |
$begingroup$
If $displaystyle bigg(frac{1+x}{1-x}bigg)^n=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots +infty,$ then value of
$(1); displaystyle frac{3b_{3}-b_{1}}{b_{2}}$
$(2); displaystyle frac{2b_{4}-b_{2}}{b_{3}}$
$(3); displaystyle frac{3b_{6}-2b_{4}}{b_{5}}$
$(4); displaystyle frac{5b_{10}-4b_{8}}{b_{9}}$
Answers given in $n,2n,3n,4n$ formats
What I tried:
$displaystyle (1+x)^n(1-x)^{-n}=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots cdots infty$
$displaystyle bigg[1+nx+frac{n(n-1)}{2}x^2+frac{n(n-1)(n-2)}{6}cdots bigg]bigg[1+nx+frac{n(n+1)}{2}x^2+frac{n(n+1)(n+2)}{6}x^3+cdots bigg]=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots $
$displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$
How do I find other coefficients? Help me, please.
binomial-theorem
$endgroup$
add a comment |
$begingroup$
If $displaystyle bigg(frac{1+x}{1-x}bigg)^n=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots +infty,$ then value of
$(1); displaystyle frac{3b_{3}-b_{1}}{b_{2}}$
$(2); displaystyle frac{2b_{4}-b_{2}}{b_{3}}$
$(3); displaystyle frac{3b_{6}-2b_{4}}{b_{5}}$
$(4); displaystyle frac{5b_{10}-4b_{8}}{b_{9}}$
Answers given in $n,2n,3n,4n$ formats
What I tried:
$displaystyle (1+x)^n(1-x)^{-n}=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots cdots infty$
$displaystyle bigg[1+nx+frac{n(n-1)}{2}x^2+frac{n(n-1)(n-2)}{6}cdots bigg]bigg[1+nx+frac{n(n+1)}{2}x^2+frac{n(n+1)(n+2)}{6}x^3+cdots bigg]=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots $
$displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$
How do I find other coefficients? Help me, please.
binomial-theorem
$endgroup$
If $displaystyle bigg(frac{1+x}{1-x}bigg)^n=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots +infty,$ then value of
$(1); displaystyle frac{3b_{3}-b_{1}}{b_{2}}$
$(2); displaystyle frac{2b_{4}-b_{2}}{b_{3}}$
$(3); displaystyle frac{3b_{6}-2b_{4}}{b_{5}}$
$(4); displaystyle frac{5b_{10}-4b_{8}}{b_{9}}$
Answers given in $n,2n,3n,4n$ formats
What I tried:
$displaystyle (1+x)^n(1-x)^{-n}=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots cdots infty$
$displaystyle bigg[1+nx+frac{n(n-1)}{2}x^2+frac{n(n-1)(n-2)}{6}cdots bigg]bigg[1+nx+frac{n(n+1)}{2}x^2+frac{n(n+1)(n+2)}{6}x^3+cdots bigg]=1+b_{1}x+b_{2}x^2+b_{3}x^3+cdots $
$displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$
How do I find other coefficients? Help me, please.
binomial-theorem
binomial-theorem
edited Feb 1 at 5:37
J. W. Tanner
4,6441420
4,6441420
asked Feb 1 at 5:03
jackyjacky
1,344816
1,344816
add a comment |
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
You made a mistake. It is not $displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$ , but
$b_{1}=2n$
$b_{2}=2n^2$
$b_{3}=frac23(n+2n^3)$
To go up to $b_{10}$, the series has to be expended up to $x^{10}$.
By brut force :
$b_{4}=frac23(2n^2+n^4)$
$b_{5}=frac{2}{15}(3n+10n^3+2n^5)$
$b_{6}=frac{2}{45}(23n^2+20n^4+2n^6)$
$b_{7}=frac{2}{315}(45n+196n^3+70n^5+4n^7)$
$b_{8}=frac{2}{315}(132n^2+154n^4+28n^6+n^8)$
$b_{9}=frac{2}{2835}(315n+1636n^3+798n^5+84n^7+2n^8)$
$b_{10}=frac{2}{14175}(5167n^2+7180n^4+1806n^6+120n^8+2n^{10})$
$frac{3b_{3}-b_{1}}{b_{2}}=2n$
Then, a supposed recurrence :
$frac{1b_{2}-0b_{0}}{b_{1}}=n$
$frac{2b_{4}-1b_{2}}{b_{3}}=n$
$frac{3b_{6}-2b_{4}}{b_{5}}=n$
$frac{4b_{8}-3b_{6}}{b_{7}}=n$
$frac{5b_{10}-4b_{8}}{b_{9}}=n$
$...$
$frac{k,b_{2k}-(k-1)b_{2k-2}}{b_{2k-1}}=n$ , not proved.
$endgroup$
add a comment |
$begingroup$
Here is a slightly different method to find the coefficients. It is convenient to use the coefficient of operator $[x^j]$ to denote the coefficient of $x^j$ in a series.
We obtain
begin{align*}
color{blue}{[x^j]left(frac{1+x}{1-x}right)^n}
&=[x^j](1-x)^{-n}(1+x)^n\
&=[x^j]sum_{k=0}^inftybinom{-n}{k}(-x)^k(1+x)^ntag{1}\
&=[x^j]sum_{k=0}^inftybinom{n+k-1}{k}x^k(1+x)^ntag{2}\
&=sum_{k=0}^jbinom{n+k-1}{k}[x^{j-k}](1+x)^ntag{3}\
&,,color{blue}{=sum_{k=0}^jbinom{n+k-1}{k}binom{n}{j-k}}tag{4}
end{align*}
Comment:
In (1) we apply the binomial series expansion
In (2) we use the binomial identity $binom{-p}{q}=binom{p+q-1}{q}(-1)^q$
In (3) we use the linearity of the coefficient of operator and apply the rule $[x^{p-q}]A(x)=[x^p]x^qA(x)$. We also set the upper limit of the sum to $j$ since other values do not contribute.
In (4) we select the coefficient of $[x^{j-k}](1+x)^n=[x^{j-k}]sum_{t=0}^nbinom{n}{t}x^t$.
$endgroup$
add a comment |
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2 Answers
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2 Answers
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$begingroup$
You made a mistake. It is not $displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$ , but
$b_{1}=2n$
$b_{2}=2n^2$
$b_{3}=frac23(n+2n^3)$
To go up to $b_{10}$, the series has to be expended up to $x^{10}$.
By brut force :
$b_{4}=frac23(2n^2+n^4)$
$b_{5}=frac{2}{15}(3n+10n^3+2n^5)$
$b_{6}=frac{2}{45}(23n^2+20n^4+2n^6)$
$b_{7}=frac{2}{315}(45n+196n^3+70n^5+4n^7)$
$b_{8}=frac{2}{315}(132n^2+154n^4+28n^6+n^8)$
$b_{9}=frac{2}{2835}(315n+1636n^3+798n^5+84n^7+2n^8)$
$b_{10}=frac{2}{14175}(5167n^2+7180n^4+1806n^6+120n^8+2n^{10})$
$frac{3b_{3}-b_{1}}{b_{2}}=2n$
Then, a supposed recurrence :
$frac{1b_{2}-0b_{0}}{b_{1}}=n$
$frac{2b_{4}-1b_{2}}{b_{3}}=n$
$frac{3b_{6}-2b_{4}}{b_{5}}=n$
$frac{4b_{8}-3b_{6}}{b_{7}}=n$
$frac{5b_{10}-4b_{8}}{b_{9}}=n$
$...$
$frac{k,b_{2k}-(k-1)b_{2k-2}}{b_{2k-1}}=n$ , not proved.
$endgroup$
add a comment |
$begingroup$
You made a mistake. It is not $displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$ , but
$b_{1}=2n$
$b_{2}=2n^2$
$b_{3}=frac23(n+2n^3)$
To go up to $b_{10}$, the series has to be expended up to $x^{10}$.
By brut force :
$b_{4}=frac23(2n^2+n^4)$
$b_{5}=frac{2}{15}(3n+10n^3+2n^5)$
$b_{6}=frac{2}{45}(23n^2+20n^4+2n^6)$
$b_{7}=frac{2}{315}(45n+196n^3+70n^5+4n^7)$
$b_{8}=frac{2}{315}(132n^2+154n^4+28n^6+n^8)$
$b_{9}=frac{2}{2835}(315n+1636n^3+798n^5+84n^7+2n^8)$
$b_{10}=frac{2}{14175}(5167n^2+7180n^4+1806n^6+120n^8+2n^{10})$
$frac{3b_{3}-b_{1}}{b_{2}}=2n$
Then, a supposed recurrence :
$frac{1b_{2}-0b_{0}}{b_{1}}=n$
$frac{2b_{4}-1b_{2}}{b_{3}}=n$
$frac{3b_{6}-2b_{4}}{b_{5}}=n$
$frac{4b_{8}-3b_{6}}{b_{7}}=n$
$frac{5b_{10}-4b_{8}}{b_{9}}=n$
$...$
$frac{k,b_{2k}-(k-1)b_{2k-2}}{b_{2k-1}}=n$ , not proved.
$endgroup$
add a comment |
$begingroup$
You made a mistake. It is not $displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$ , but
$b_{1}=2n$
$b_{2}=2n^2$
$b_{3}=frac23(n+2n^3)$
To go up to $b_{10}$, the series has to be expended up to $x^{10}$.
By brut force :
$b_{4}=frac23(2n^2+n^4)$
$b_{5}=frac{2}{15}(3n+10n^3+2n^5)$
$b_{6}=frac{2}{45}(23n^2+20n^4+2n^6)$
$b_{7}=frac{2}{315}(45n+196n^3+70n^5+4n^7)$
$b_{8}=frac{2}{315}(132n^2+154n^4+28n^6+n^8)$
$b_{9}=frac{2}{2835}(315n+1636n^3+798n^5+84n^7+2n^8)$
$b_{10}=frac{2}{14175}(5167n^2+7180n^4+1806n^6+120n^8+2n^{10})$
$frac{3b_{3}-b_{1}}{b_{2}}=2n$
Then, a supposed recurrence :
$frac{1b_{2}-0b_{0}}{b_{1}}=n$
$frac{2b_{4}-1b_{2}}{b_{3}}=n$
$frac{3b_{6}-2b_{4}}{b_{5}}=n$
$frac{4b_{8}-3b_{6}}{b_{7}}=n$
$frac{5b_{10}-4b_{8}}{b_{9}}=n$
$...$
$frac{k,b_{2k}-(k-1)b_{2k-2}}{b_{2k-1}}=n$ , not proved.
$endgroup$
You made a mistake. It is not $displaystyle b_{1}=n,b_{2}=n,b_{3}=4n^2$ , but
$b_{1}=2n$
$b_{2}=2n^2$
$b_{3}=frac23(n+2n^3)$
To go up to $b_{10}$, the series has to be expended up to $x^{10}$.
By brut force :
$b_{4}=frac23(2n^2+n^4)$
$b_{5}=frac{2}{15}(3n+10n^3+2n^5)$
$b_{6}=frac{2}{45}(23n^2+20n^4+2n^6)$
$b_{7}=frac{2}{315}(45n+196n^3+70n^5+4n^7)$
$b_{8}=frac{2}{315}(132n^2+154n^4+28n^6+n^8)$
$b_{9}=frac{2}{2835}(315n+1636n^3+798n^5+84n^7+2n^8)$
$b_{10}=frac{2}{14175}(5167n^2+7180n^4+1806n^6+120n^8+2n^{10})$
$frac{3b_{3}-b_{1}}{b_{2}}=2n$
Then, a supposed recurrence :
$frac{1b_{2}-0b_{0}}{b_{1}}=n$
$frac{2b_{4}-1b_{2}}{b_{3}}=n$
$frac{3b_{6}-2b_{4}}{b_{5}}=n$
$frac{4b_{8}-3b_{6}}{b_{7}}=n$
$frac{5b_{10}-4b_{8}}{b_{9}}=n$
$...$
$frac{k,b_{2k}-(k-1)b_{2k-2}}{b_{2k-1}}=n$ , not proved.
answered Feb 1 at 7:27
JJacquelinJJacquelin
45.6k21857
45.6k21857
add a comment |
add a comment |
$begingroup$
Here is a slightly different method to find the coefficients. It is convenient to use the coefficient of operator $[x^j]$ to denote the coefficient of $x^j$ in a series.
We obtain
begin{align*}
color{blue}{[x^j]left(frac{1+x}{1-x}right)^n}
&=[x^j](1-x)^{-n}(1+x)^n\
&=[x^j]sum_{k=0}^inftybinom{-n}{k}(-x)^k(1+x)^ntag{1}\
&=[x^j]sum_{k=0}^inftybinom{n+k-1}{k}x^k(1+x)^ntag{2}\
&=sum_{k=0}^jbinom{n+k-1}{k}[x^{j-k}](1+x)^ntag{3}\
&,,color{blue}{=sum_{k=0}^jbinom{n+k-1}{k}binom{n}{j-k}}tag{4}
end{align*}
Comment:
In (1) we apply the binomial series expansion
In (2) we use the binomial identity $binom{-p}{q}=binom{p+q-1}{q}(-1)^q$
In (3) we use the linearity of the coefficient of operator and apply the rule $[x^{p-q}]A(x)=[x^p]x^qA(x)$. We also set the upper limit of the sum to $j$ since other values do not contribute.
In (4) we select the coefficient of $[x^{j-k}](1+x)^n=[x^{j-k}]sum_{t=0}^nbinom{n}{t}x^t$.
$endgroup$
add a comment |
$begingroup$
Here is a slightly different method to find the coefficients. It is convenient to use the coefficient of operator $[x^j]$ to denote the coefficient of $x^j$ in a series.
We obtain
begin{align*}
color{blue}{[x^j]left(frac{1+x}{1-x}right)^n}
&=[x^j](1-x)^{-n}(1+x)^n\
&=[x^j]sum_{k=0}^inftybinom{-n}{k}(-x)^k(1+x)^ntag{1}\
&=[x^j]sum_{k=0}^inftybinom{n+k-1}{k}x^k(1+x)^ntag{2}\
&=sum_{k=0}^jbinom{n+k-1}{k}[x^{j-k}](1+x)^ntag{3}\
&,,color{blue}{=sum_{k=0}^jbinom{n+k-1}{k}binom{n}{j-k}}tag{4}
end{align*}
Comment:
In (1) we apply the binomial series expansion
In (2) we use the binomial identity $binom{-p}{q}=binom{p+q-1}{q}(-1)^q$
In (3) we use the linearity of the coefficient of operator and apply the rule $[x^{p-q}]A(x)=[x^p]x^qA(x)$. We also set the upper limit of the sum to $j$ since other values do not contribute.
In (4) we select the coefficient of $[x^{j-k}](1+x)^n=[x^{j-k}]sum_{t=0}^nbinom{n}{t}x^t$.
$endgroup$
add a comment |
$begingroup$
Here is a slightly different method to find the coefficients. It is convenient to use the coefficient of operator $[x^j]$ to denote the coefficient of $x^j$ in a series.
We obtain
begin{align*}
color{blue}{[x^j]left(frac{1+x}{1-x}right)^n}
&=[x^j](1-x)^{-n}(1+x)^n\
&=[x^j]sum_{k=0}^inftybinom{-n}{k}(-x)^k(1+x)^ntag{1}\
&=[x^j]sum_{k=0}^inftybinom{n+k-1}{k}x^k(1+x)^ntag{2}\
&=sum_{k=0}^jbinom{n+k-1}{k}[x^{j-k}](1+x)^ntag{3}\
&,,color{blue}{=sum_{k=0}^jbinom{n+k-1}{k}binom{n}{j-k}}tag{4}
end{align*}
Comment:
In (1) we apply the binomial series expansion
In (2) we use the binomial identity $binom{-p}{q}=binom{p+q-1}{q}(-1)^q$
In (3) we use the linearity of the coefficient of operator and apply the rule $[x^{p-q}]A(x)=[x^p]x^qA(x)$. We also set the upper limit of the sum to $j$ since other values do not contribute.
In (4) we select the coefficient of $[x^{j-k}](1+x)^n=[x^{j-k}]sum_{t=0}^nbinom{n}{t}x^t$.
$endgroup$
Here is a slightly different method to find the coefficients. It is convenient to use the coefficient of operator $[x^j]$ to denote the coefficient of $x^j$ in a series.
We obtain
begin{align*}
color{blue}{[x^j]left(frac{1+x}{1-x}right)^n}
&=[x^j](1-x)^{-n}(1+x)^n\
&=[x^j]sum_{k=0}^inftybinom{-n}{k}(-x)^k(1+x)^ntag{1}\
&=[x^j]sum_{k=0}^inftybinom{n+k-1}{k}x^k(1+x)^ntag{2}\
&=sum_{k=0}^jbinom{n+k-1}{k}[x^{j-k}](1+x)^ntag{3}\
&,,color{blue}{=sum_{k=0}^jbinom{n+k-1}{k}binom{n}{j-k}}tag{4}
end{align*}
Comment:
In (1) we apply the binomial series expansion
In (2) we use the binomial identity $binom{-p}{q}=binom{p+q-1}{q}(-1)^q$
In (3) we use the linearity of the coefficient of operator and apply the rule $[x^{p-q}]A(x)=[x^p]x^qA(x)$. We also set the upper limit of the sum to $j$ since other values do not contribute.
In (4) we select the coefficient of $[x^{j-k}](1+x)^n=[x^{j-k}]sum_{t=0}^nbinom{n}{t}x^t$.
answered Feb 1 at 20:42
Markus ScheuerMarkus Scheuer
64.1k460152
64.1k460152
add a comment |
add a comment |
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