Answer: ABCD is a quadrilateral
To prove : ∠AOB=
2
1
(∠C+∠D)
AO and BO is bisector of A and B
∠1=∠2∠3=∠4...(1)
∠A+∠B+∠C+∠D=360
(Angle sum property)
2
1
(∠A+∠B+∠C+∠D)=180...(2)
In △AOB
∠1+∠3+∠5=
2
1
(∠A+∠B+∠C+∠D)
∠1+∠3+∠5=∠1+∠3+
2
1
(∠C+∠D)
∠AOB=
2
1
(∠C+∠D)
Explanation: In a quadrilateral ABCD. AO and BO are bisectors of angle A and angle B respectively. Prove that ∠AOB=
2
1
{∠C+∠D}.
