Ok here is my attempt.

Suppose that is rational. Therefore = for some integers m, n with n 0.

We may suppose without loss of generality (wlog) that m, n have no common factor (i.e. are coprime). Now = and so is even. Put . Then = and so =

which implies that n is even. This contradicts the assumption that m, n are coprime.

Hence must be irrational.

Is this correct?