Moment of inertia of a thin square plate about an axis passing through its diagonal is I. Its moment of inertia about an axis passing through its centre in the plane of the plate and making an angle θ with the diagonal is
IBD = I = IAC
By perpendicular axis Theorem
IBD + IAC = IZ ⇒ 2I = IZ ......(1)
Since I1 and I2 are mutually perpendicular and I1 = I2
by perpendicular axis Theorem I1 + I2 = Iz = 2I
2I1 = 2I ⇒ I1 = I