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Three identical thin rods, each of length L and mass m, are welded perpendicular to one another. The assembly is rotated about an axis that passes through the end of one rod and is parallel to another. Determine the moment of intertia of this structure.

Ok what I know is that the rod which is connected to the axis of rotation has inertia of I = 1/3ML^2

And I know that the rod which is parallel to the axis has inertia of I = ML^2

So far I have (1/3ML^2) + (ML^2) and I just need one other moment of intertia which I am having difficulty finding.

I believe I need to apply the parallel-axis theorem, but I dont understand why.

If someone could please explain why you would use this theorem it would be very helpful! Thanks!