Ta có: `S_{AOB}+S_{AOD}=S_{ABD}`
`=1/2AO.OB sin AOB + 1/2 AO*OD .sinAOD`
`=1/2AO*BDsin50^0`
Lại có: `S_{DBC}=S_{AOC}+S_{OCD}=1/2BO*OC*sinBOC+1/2OC*CDsinCOD`
`=1/2CO*BD*sin50^0`
`=>S_{BCD}=S_{ABD}+S_{BCD}=1/2BD*sin50^0(OA+OC)`
`=1/2BD*AC*sin50^0`
`=7,66cm^2`