DSE 數學 必修部分 練習卷 第21題

Angle at center of arc AB + arc BC + arc CD = 360 - 70 = 290 degree.
Angle at center of arc AB = 290 x ( 2/5) = 116 degree.
So angle at center of arc BOD = 116 + 70 = 186 degree
So angle BCD = half angle at center = 186/2 = 93 degree ( answer is C).

2013-09-01 13:21:57 補充：
Remark : 2/5 = 2/(2 + 2 + 1)