[en] In this thesis the measurement of inclusive dijet and trijet cross sections in deep-inelastic ep scattering at HERA is presented. The kinematic phase space of the measurement was defined by 125< Q²<20000 GeV² and 0.2< y<0.6, where Q² and y are the virtuality and the inelasticity, respectively. The data sample was taken during the years 1998-2000 and 2004-2007 with the ZEUS detector and corresponded to an integrated luminosity of 374 pb^-1. The inclusive k_t jet algorithm was applied to the massless final-state objects in the Breit reference frame. The cross sections referred to jets with E_T,B^jet>8 GeV and -1<η_LAB^jet<2.5, where the first quantity is the transverse jet energy in the Breit frame and the latter the jet pseudorapidity in the laboratory frame. For the selection of dijet (trijet) events it was required that at least the two (three) highest-transverse-energy jets have exceeded the transverse-energy threshold. Additionally, the invariant dijet mass of the two highest-transverse-energy jets in the event was required to be greater than 20 GeV. The measurements were compared to fixed-order NLO QCD calculations as implemented in the NLOJET++ program. The agreement in shape and normalisation between theory and the measurement was good. The ratio, R_3/2, between the cross sections for trijet and dijet production was determined as a function of the average transverse jet energy in the Breit frame, E_T,B^jet, in intervals of Q². The quantity R_3/2 was utilised for an extraction of the strong coupling, α_s, with partially reduced systematic uncertainties. The extracted value was in agreement with the world average value of α_s. In a second part, test-beam measurements were performed with the EUDET pixel telescope. During the work for this thesis, the online-monitoring software was improved, the MIMOSA 26 sensors were integrated into the offline analysis software and the first data taken with these sensors were analysed. The first data were taken with the demonstrator telescope together with three MIMOSA 26 sensors that were operated as devices-under-test. The second data sample was taken with a telescope that consisted of six MIMOSA 26 sensors, of which five could be used. The single-point resolution and the detection efficiency were determined and found to be consistent with the expectation. (orig.)