Consider the disc in figure:
we know M.I. of disc about an axis perpendicular to disc and passing through its CM = \(\frac {MR^2}{2}\)
Using perpendicular axis theorem
lAB + ICD = ||⊥ = \(\frac {MR^2}{2}\)
⇒ 2 IAB = \(\frac {MR^2}{2}\) [Since IAB = ICD] (symmetry)
∴ IAB = \(\frac {MR^2}{2}\)
using parallel axis theorem