Kẻ AK, IH vuông với AB, DC
Ta có: S(ADC) = S(ADO) +S(ODC) = 40 +80 =120 (cm^2)
Ta có: S(ODC)/S(ADC) = (1/2.OH.DC)/(1/2 .AK. DC) = 80/120 = 2/3
=> OH/AK = 2/3
mà OH + OI = HI = AK => OI/HI=1/3
=> AB/DC = 1/3
Ta có: S(ADC) / S(ABCD) = (1/2.AK.DC)/(1/2(AB+CD).HI)= (1/2.HI.DC)/(1/2(1/3CD+CD).HI) = 3/4
=> S(ABCD) = S(ADC): 3/4 = 120 : 3/4 = 160 cm^2