正确：设对角线AC,BD交于O
∵AC⊥BD
∴S△ABC=1/2AC×OB
S△ADC=1/2AC×OD
∵OB+OD=BD
∴S四边形ABCD=S△ABC+S△ADC
=1/2AC×OB+1/2AC×OD
=1/2AC(OB+OD)
=1/2ac×bd