DO =BO ⇒<DBO =<BDO || пусть || = α;
DO =OC ⇒< DCO =<CDO || пусть || = β ;
ΔBDC :
<DBO + < DCO +<BDC =180°; * * * * * <BDC =<BDO + <CDO =α +β * * * * *
α +β + (α +β ) =180° ;
2(α +β) =180° ;
α +β =90° .
<BDC = 90° .
