AD//BC,△AOD∽△COB,S△AOD/S△COB=（AO/CO）^2=(1/2)^2=1/4,S△COB=4S△AOD=40(cm^2),△ABO和△BOC共用同一高,S△ABO/S△BOC=AO/CO=1/2,S△ABO=S△BOC/2=20（cm^2),S△CDO=S△ABO=20(cm^2),∴S梯形ABCD=S△AOD+S△...