The moment of inertia of a planar square about a planar axis parallel to one side is 10kgm2. What is the moment of inertia about a diagonal?

Correct Answer: 10kgm2
Using perpendicular axis theorem we can say that sum of moment of inertia about two perpendicular axes will be the same as sum of moment of inertia about other 2 perpendicular axes. Let I1 be the moment of inertia about a diagonal, we can say: I1 + I1= 10 + 10 = 20kgm2 Or I1 =10kgm2.