$$\require{cancel}$$
The second moment of inertia of any body can be written in the form $$mk^2$$. Thus, for the rod, the disc (about an axis perpendicular to its plane), the triangle and the disc (about a diameter), $$k$$ has the values
$$\dfrac{l}{\sqrt{3}} = 0.866l, \dfrac{a}{\sqrt{2}} = 0.707a, \dfrac{a}{\sqrt{6}} = 0.408a, \dfrac{a}{2} = 0.500a$$
$$k$$ is called the radius of gyration. If you were to concentrate all the mass of a body at its radius of gyration, its moment of inertia would remain the same.