Question
In figure, the bisectors of ∠ABC and ∠BCA intersect each other at the point O. Prove that 
Solution
∵ BO is the bisector of ∠ABC.
∵ CO is the bisector of ∠ACB
∵ CO is the bisector of ∠ACB
In ∆OBC, ∠BOC + ∠OBC + ∠OCB = 180°
| ∵ The sum of the three angles of a ∆ is 180°
| From (1) and (2)
In ∠ABC, ∠A + ∠B + ∠C = 180°
| ∵ The sum of the three angles of a triangle is 180°
From (3) and (4), we have
