Measurements of CP observables in B → DK decays are presented, where D represents a superposition of D and D¯ states. The D meson is reconstructed in the two-body final states Kπ, πK, KK and ππ, and, for the first time, in the fourbody final states Kπππ, πKππ and ππππ. The analysis uses a sample of neutral B mesons produced in proton-proton collisions, corresponding to an integrated luminosity of 1.0, 2.0 and 1.8 fb collected with the LHCb detector at centre-of-mass energies of s=7,8 and 13 TeV, respectively. First observations of the decays B → D(πK)K and B → D(ππππ)K are obtained. The measured observables are interpreted in terms of the CP -violating weak phase γ. [Figure not available: see fulltext.]

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