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Algebra Level 3

True or False?

\quad For all even positive integers n > 1 n> 1 , ( 3 + 2 ) n + ( 3 2 ) n (\sqrt3 + \sqrt2)^n + (\sqrt3 - \sqrt2)^n is always an integer.

False True

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1 solution

Michael Fuller
Jul 20, 2016

Using the binomial expansion, ( 3 + 2 ) n + ( 3 2 ) n (\sqrt3+\sqrt2)^n+(\sqrt3-\sqrt2)^n = [ 3 n + ( n 1 ) 3 n 1 2 + ( n 2 ) 3 n 2 2 2 + + 2 n ] + [ 3 n ( n 1 ) 3 n 1 2 + ( n 2 ) 3 n 2 2 2 + + 2 n ] = \left[ \sqrt3^n + \begin{pmatrix} n \\ 1 \end{pmatrix} \sqrt3^{n-1}\sqrt2 + \begin{pmatrix} n \\ 2 \end{pmatrix} \sqrt3^{n-2}\sqrt2^2+ \cdots + \sqrt2^n\right] + \left[ \sqrt3^n - \begin{pmatrix} n \\ 1 \end{pmatrix} \sqrt3^{n-1}\sqrt2 + \begin{pmatrix} n \\ 2 \end{pmatrix} \sqrt3^{n-2}\sqrt2^2+ \cdots + \sqrt2^n\right] = 2 [ 3 n + ( n 2 ) 3 n 2 2 2 + ( n 4 ) 3 n 4 2 4 + + 2 n ] = 2 \left[ \sqrt3^n + \begin{pmatrix} n \\ 2 \end{pmatrix} \sqrt3^{n-2}\sqrt2^2 + \begin{pmatrix} n \\ 4 \end{pmatrix} \sqrt3^{n-4}\sqrt2^4 + \cdots + \sqrt2^n \right] Since n n is an even positive integer, we can write n = 2 a n=2a for some integer a a : = 2 [ 3 2 a + ( n 2 ) 3 2 a 2 2 2 + ( n 4 ) 3 2 a 4 2 4 + + 2 2 a ] = 2 \left[ \sqrt3^{2a} + \begin{pmatrix} n \\ 2 \end{pmatrix} \sqrt3^{2a-2}\sqrt2^2 + \begin{pmatrix} n \\ 4 \end{pmatrix} \sqrt3^{2a-4}\sqrt2^4 + \cdots + \sqrt2^{2a} \right] = 2 [ 3 a + ( n 2 ) 3 a 1 2 + ( n 4 ) 3 a 2 2 2 + + 2 a ] = 2 \left[ 3^{a} + \begin{pmatrix} n \\ 2 \end{pmatrix} 3^{a-1} 2 + \begin{pmatrix} n \\ 4 \end{pmatrix} 3^{a-2}2^2 + \cdots + 2^a \right] which is an integer. Therefore the statement is True \large \color{#20A900}{\boxed{\text{True}}} .

I think you meant to say n \textit{n} is any positive even \textbf{even} integer, since you stated that in your solution. It is definitely false if odd integers are included.

Richard Costen - 4 years, 11 months ago

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I just found out that LaTex doesn't work in comments. :)

Richard Costen - 4 years, 11 months ago

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@Richard Costen LaTeX does work in Comments

Example : 1 0 12000 + n × k ! \lfloor 10^{12000}+n \times k! \rfloor

Mehul Arora - 4 years, 11 months ago

Same method.

Mehul Arora - 4 years, 11 months ago

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