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?