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We study magnetoresistivity oscillations induced by microwave radiation or acoustic phonons in high-mobility two-dimensional electron systems subject to dc electric field. In microwave-irradiated samples the response is governed by combined electron transitions, composed of microwave absorption and scattering off impurities. In non-irradiated samples, acoustic phonon resonances are tuned by dc electric field. Here, we show that in both experiments scattering off impurities without microwave or phonon absorption plays an important role and might even dominate the response.