suppose that \(\sqrt{2}=\dfrac{a}{b}\) \(\in Q\)  (*)

\(\Rightarrow a^2=2b^2\)\(\Rightarrow a^2\) is a even \(\Rightarrow a\) is a even \(\Rightarrow b\) is a even (!*).

78:16<=n: is a min integer

n=5