In the given figure, ΔABC and ΔOBC are both isosceles.
It can be concluded that:
ΔAOB≅ΔAOC
∠ABO=∠ACO
In ΔAOB and ΔAOC,AB=AC(Sides of isosceles ΔABC).OB=OC(Sides of isosceles ΔOBC).AO is common∴ΔAOB≅ΔAOC(by SSS congruence)
And ∠ABO=∠ACO ( By CPCT)