Features of resonance power absorption in a lossy dielectric resonator shaped as a rectangular trapezium in a rectangular waveguide are studied. It is found that such a resonator can provide strong absorption (or reflection) of the TE10-mode input from the straight (or inclined) interface port. The maximum absorption is observed when the sum of the coupling coefficients equals unity. Simple formulas relating the scattering and absorption coefficients with the coupling coefficients at the resonance frequency are presented. The physical mechanism of resonance absorption is discussed.