Sum rules for B-->pi eta('), K eta('), eta(') eta(') decays

We provide a set of sum rules, using flavour symmetries, relating CP-averaged ratios and CP asymmetries of different neutral and charged B mesons decaying into an eta(') particle together with a pion, a kaon or a second eta('). These sume rules allow us to give a prediction for the B^0-->K^0 eta branching ratio. We also predict a clear sign anti-correlation between the two B^0-->pi^0 (eta,eta') CP-asymmetries, and find a combined constraint on the branching ratios and CP-asymmetries of the three B-->eta(')eta(') decay modes.