Prove that the coefficient of $x^{n}$ in the expansion of $(1+x)^{2n}$ is twice the coefficient of $x^{n}$ in the expansion of $(1+x)^{2 n-1}$
It is known that $(r+1)^{\text {th }}$ term, $\left(T_{r+1}\right),$ in the binomial expansion of $(a+b)^{n}$ is given by
${T_{r + 1}} = {\,^n}{C_r}{a^{n - r}}{b^r}$
Assuming that $x^{n}$ occurs in the $(r+1)^{\text {th }}$ term of the expansion of $(1+x)^{2 n}$, we obtain
${T_{r + 1}} = {\,^{2n}}{C_r}{(1)^{2n - 1}}{(x)^r} = {\,^{2n}}{C_r}{(x)^r}$
Comparing the indices of $x$ in $x^{n}$ and in $T_{r+1},$ we obtain $r=n$
Therefore, the coefficient of $x^{n}$ in the expansion of $(1+x)^{2 n}$ is
$^{2n}{C_n} = \frac{{(2n)!}}{{n!(2n - n)!}} = \frac{{(2n)!}}{{n!n!}} = \frac{{(2n)!}}{{{{(n!)}^2}}}$ ...........$(1)$
Assuming that $x^{n}$ occurs in the $(k+1)^{\text {th }}$ term of the expansion of $(1+x)^{2 n-1}$, we obtain
${T_{k + 1}} = {\,^{2n}}{C_k}{(1)^{2n - 1 - k}}{(x)^k} = {\,^{2n}}{C_k}{(x)^k}$
Comparing the indices of $x$ in $x^{n}$ and in $T_{k+1},$ we obtain $k=n$
Therefore, the coefficient of $x^{n}$ in the expansion of $(1+x)^{2 n-1}$ is
${\,^{2n - 1}}{C_n} = \frac{{(2n - 1)!}}{{n!(2n - 1 - n)!}} = \frac{{(2n - 1)!}}{{n!(n - 1)!}}$
$ = \frac{{2n \cdot (2n - 1)!}}{{2n.n!(n - 1)!}} = \frac{{(2n)!}}{{2n!n!}} = \frac{1}{2}\left[ {\frac{{(2n)!}}{{{{(n!)}^2}}}} \right]$ ............$(2)$
From $(1)$ and $(2),$ it is observed that
$\frac{1}{2}{\rm{(}}{\,^{2n}}{C_n}{\rm{)}} = {\,^{2n - 1}}{C_n}$
$ \Rightarrow {\,^{2n}}{C_n} = 2{\rm{(}}{\,^{2n - 1}}{C_n}{\rm{)}}$
Therefore, the coefficient of $x^{n}$ expansion of $(1+x)^{2 n}$ is twice the coefficient of expansion of $x^{n}$ in the $(1+x)^{2 n-1}$
Hence proved.
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