Mass per unit area \(=\) \(\mu\)
Total mass \(M=\mu l^{2}\)
Now, the moment of inertia is \(I=\frac{M R^{2}}{12}\)
By perpendicular axis theorem \(I=I_{x}+I_{y}\)
\(I=2 I_{x}\)
\(I=2 \times \frac{\mu l^{2} \times l^{2}}{12}\)
\(I=\frac{\mu l^{4}}{6}\)
Hence, the moment of inertia of a square sheet is \(\frac{\mu l^{4}}{6}\)