∵OD⊥AB OE⊥AC
∴D为AB中点,E为AC中点(垂径定理)
∵AB=AC
∴AD=CE
∵同圆或等圆中,同弦所对弦心距相等,
∴OD=OE
1/2OD*AB=1/2OE*AC
∴ S△AOB=S△AOC