Theorem Of Perpendicular Axis
It state that the moment of inertia of a planner body about an axis perpendicular to its plane is equal to the sum of its moments of inertia about two perpendicular axis concurrent with perpendicular axis and lying in the plane of the body.
From the definition of moment of inertia at point P along x-axis
Theorem Of Parallel Axis
It state that the moment of inertia of a body about an axis is equal to the sum of moments of inertia of the body about a parallel axis passing through its centre of mass and the product of its mass and the square of distance between the two parallel axis.