∴∠C=80 -50 =30,
∴∠abd=∠c=30;
(2) If point O is OE⊥BD of point E, BD=2BE,
∵∠ABD=30,OB=5cm,
∴BE=OB? cos30 =3×32=332,
∴BD=2BE=2×332=33cm.