`∆BOA~∆DOC`
`=>(S_(AOD))/(S_(AOB))=(OD)^2/(OB)^2`
`=>(OD)/(OB)=2020/2019`
Có `(S_(AOD))/(S_(AOB))=(OD)/(OB)=2020/2019`
(Kẻ đường cao sẽ thấy.)
`=>S_(AOD)=2020.2019 cm^2`
CMTT : `S_(BOC)=2020.2019 cm^2`
`=>S_(ABCD)=2020^2+2019^2+2.2020.2019=(2020+2019)^2=4039^2 cm^2`