∵△AOD和△ABO在BD上等高
∴S△AOD/S△ABO=OD/OB=20/40=1/2
∵S△ACD=S△ABD
即S△ABO+S△AOD=S△AOD+S△DCO
那么S△ABO=S△DCO=40
∴S△DCO/S△BOC=OD/OB=1/2
S△BOC=2S△DCO=80
∴等腰梯形面积
=S△AOD+S△ABO+S△DCO+S△BOC
=20+40+40+80
=180
∵△AOD和△ABO在BD上等高
∴S△AOD/S△ABO=OD/OB=20/40=1/2
∵S△ACD=S△ABD
即S△ABO+S△AOD=S△AOD+S△DCO
那么S△ABO=S△DCO=40
∴S△DCO/S△BOC=OD/OB=1/2
S△BOC=2S△DCO=80
∴等腰梯形面积
=S△AOD+S△ABO+S△DCO+S△BOC
=20+40+40+80
=180