Find n, if the ratio of the fifth term from the beginning to the fifth term from the end in the expansion of
Find n, if the ratio of the fifth term from the beginning to the fifth term from the end in the expansion of $\left(\sqrt[4]{2}+\frac{1}{\sqrt[4]{3}}\right)^{n}$ is $\sqrt{6}: 1$
In the expansion, $(a+b)^{n}={ }^{n} C_{v} a^{n}+{ }^{n} C_{1} a^{n-1} b+{ }^{n} C_{2} a^{n-2} b^{2}+\ldots+{ }^{n} C_{n-1} a b^{n-1}+{ }^{n} C_{n} b^{n}$,
Fifth term from the beginning $={ }^{n} \mathrm{C}_{4} \mathrm{a}^{n-4} \mathrm{~b}^{4}$
Fifth term from the end $={ }^{n} \mathrm{C}_{n-4} \mathrm{a}^{4} \mathrm{~b}^{n-4}$
Therefore, it is evident that in the expansion of $\left(\sqrt[4]{2}+\frac{1}{\sqrt[4]{3}}\right)^{n}$, the fifth term from the beginning is ${ }^{n} C_{4}(\sqrt[4]{2})^{n-4}\left(\frac{1}{\sqrt[4]{3}}\right)^{4}$ and the fifth term from the end is ${ }^{n} C_{n-4}(\sqrt[4]{2})^{4}\left(\frac{1}{\sqrt[4]{3}}\right)^{n-4}$.
${ }^{n} C_{4}(\sqrt[4]{2})^{n-4}\left(\frac{1}{\sqrt[4]{3}}\right)^{4}={ }^{n} C_{4} \frac{(\sqrt[4]{2})^{n}}{(\sqrt[4]{2})^{4}} \cdot \frac{1}{3}={ }^{n} C_{4} \frac{(\sqrt[4]{2})^{n}}{2} \cdot \frac{1}{3}=\frac{n !}{6.4 !(n-4) !}(\sqrt[4]{2})^{n}$ (1)
${ }^{n} C_{n-4}(\sqrt[4]{2})^{4}\left(\frac{1}{\sqrt[4]{3}}\right)^{n-4}={ }^{n-C} C_{n-4} \cdot 2 \cdot \frac{(\sqrt[4]{3})^{4}}{(\sqrt[4]{3})^{n}}={ }^{n} C_{n-4} \cdot 2 \cdot \frac{3}{(\sqrt[4]{3})^{0}}=\frac{6 n !}{(n-4) ! 4 !} \cdot \frac{1}{(\sqrt[4]{3})^{n}}$ (2)
It is given that the ratio of the fifth term from the beginning to the fifth term from the end is $\sqrt{6}: 1$. Therefore, from (1) and (2), we obtain
$\frac{n !}{6.4 !(n-4) !}(\sqrt[4]{2})^{n}: \frac{6 n !}{(n-4) ! 4 !} \cdot \frac{1}{(\sqrt[4]{3})^{n}}=\sqrt{6}: 1$
$\Rightarrow \frac{(\sqrt[4]{2})^{n}}{6}: \frac{6}{(\sqrt[4]{3})^{n}}=\sqrt{6}: 1$
$\Rightarrow \frac{(\sqrt[4]{2})^{n}}{6} \times \frac{(\sqrt[4]{3})^{n}}{6}=\sqrt{6}$
$\Rightarrow(\sqrt[4]{6})^{n}=36 \sqrt{6}$
$\Rightarrow 6^{\frac{n}{4}}=6^{\frac{5}{2}}$
$\Rightarrow \mathrm{n}=4 \times \frac{5}{2}=10$
Thus, the value of $n$ is 10 .
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