Question

How did you get 1/2MR^2?

Answer 1

We have Moment of Inertia of a Circle about an axis Passing through its Centre and Perpendicular to its Plane or About a Transverse Axis

We have a uniform thin circular disc of mass M and radius R. Its moment of inertia about an axis passing through its centre and perpendicular to its plane is (MR²)/(2)

I    =  (MR²)/(2)