△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
如图？在哪儿？求……啥？
