△boc中, ∠boc = 180°- ∠obc - ∠ocb = 180°- ∠abc/2 - ∠acb/2 = 180°- (∠abc + ∠acb)/2 = 180°- (180°- ∠bac)/2 = 180°- (180° - α)/2 = 180°- 90° + α/2 = 90° + α/2
