  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.
