∠AOC+∠BOC=180°,
∵OD平分∠AOC,∴∠DOC=1/2∠AOC
同理∠COE=1/2∠BOC
∠DOE=∠DOC+∠COE=1/2∠AOC+1/2∠BOC=1/2(∠AOC+∠BOC)=1/2*180°
=90°
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