The angular distributions of the rare decays $B^+ to K^+mu^+mu^-$ and $B^0 to K^0_{rmscriptscriptstyle S}mu^+mu^-$ are studied with data corresponding to 3$~$fb$^{-1}$ of integrated luminosity, collected in proton-proton collisions at 7 and 8$~$TeV centre-of-mass energies with the LHCb detector. The angular distribution is described by two parameters, $F_{rm H}$ and the forward-backward asymmetry of the dimuon system $A_{rm FB}$, which are determined in bins of the dimuon mass squared. The parameter $F_{rm H}$ is a measure of the contribution from (pseudo)scalar and tensor amplitudes to the decay width. The measurements of $A_{rm FB}$ and $F_{rm H}$ reported here are the most precise to date and are compatible with predictions from the Standard Model.
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