Given:
$$ PA$$ & $$PB$$ are two tangents drawn from P to a circle with centre O at A & B respectively.
$$\angle APB={ 60 }^{ o }$$
To find out-
$$ \angle AOB$$
Solution-
$$ PA$$ & $$PB$$ are two tangents drawn from P to the circle at A & B, respectively.
$$ \therefore PA=PB\Longrightarrow \Delta PAB$$ is isosceles.
i.e $$\angle PAB=\angle PBA.$$
or $$\angle PAB+\angle PBA=2\angle PAB$$ .....(i)
Now, $$\angle PAB+\angle PBA+\angle APB={ 180 }^{ o }$$ ....(angle sum property of triangles)
$$ \Longrightarrow \angle PAB+\angle PBA{ +60 }^{ o }={ 180 }^{ O }$$
$$\Longrightarrow 2\angle PAB={ 120 }^{ o }$$ ...(from i)
$$ \therefore \angle PAB={ 60 }^{ o }=\angle PBA$$ .........(ii)
Again, $$\angle OAP={ 90 }^{ o }$$ ....(angle between a radius and the tangent at the point of contact.)
$$ \therefore \angle OAB=\angle OAP-\angle PAB={ 90 }^{ o }-{ 60 }^{ o }$$ .... (from ii) .........(iii)
Since, $$OA=OB$$ ...(radii of the same circle)
$$ \therefore \Delta OAB$$ is isosceles
$$\Longrightarrow \angle OAB=\angle OBA={ 30 }^{ o }$$ ....(from iii)
So, $$\angle AOB={ 180 }^{ o }-(\angle OAB+\angle OBA)={ 180 }^{ o }-{ 30 }^{ o }-{ 30 }^{ O }={ 120 }^{ O }$$ ...(angle sum property of triangles)