Prove that 3√2 is irrational without using prime factorization.
The standard proof that 3√2 is irrational uses prime factorization in an essential way. So I wondered if there is a proof that does not use it.
This was inspired by the fact that I know two proofs that √2 is irrational that do not use prime factorization.
The first uses
√2=√2√2−1√2−1=2−√2√2−1
to show that if √2=ab then
√2=2−abab−1=2b−aa−b
is a rational √2 with a smaller denominator.
The second uses
(x2−2y2)2=(x2+2y2)2−2(2xy)2
and 32−2⋅22=1 to show that x2−2y2=1 has arbitrarily large solutions and this contradicts √2 being rational.
I have not been able to extend either of these proofs to 3√2. Results that I do not consider "legal" in solving this problem include Fermat's Last Theorem (which definitely uses unique factorization) and the rational root theorem (which uses unique factorization
in its proof).
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