∠ABC=100°\angle ABC=100\degree∠ABC=100°
∠ACB=20°\angle ACB=20\degree∠ACB=20°
CECECE 平分 ∠ACB\angle ACB∠ACB
∠CBD=20°\angle CBD=20\degree∠CBD=20°
求 ∠CED\angle CED∠CED
