Three particles of each mass 100g are placed at the vertices of an equilateral triangle of side length 10cm. Find the moment of inertia of the system about an axis passing through the centroid of the triangle and perpendicular to its plane. I know how to solve this question but i was stucked at how to find the radius r=? Formula to find moment of inertia =3mr square.
similarly can u give the explanation of the same in case of a square to find r=?
I'm very thankful for ur help for sparing a little time for me.

Question

Three particles of each mass 100g are placed at the vertices of an equilateral triangle of side length 10cm. Find the moment of inertia of the system about an axis passing through the centroid of the triangle and perpendicular to its plane. I know how to solve this question but i was stucked at how to find the radius r=? Formula to find moment of inertia =3mr square. 
similarly can u  give the explanation of the same in case of a square to find r=? 
I'm very thankful for ur help for sparing a little time for me.

anonymous · Answer

Triangle centroid: intersection of lines from vertices to midpoints of the sides opposite them.  Square centroid: center, intersection of diagonal.

Draw these lines, find trigonometric relations that ell you how far the vertices are from the centroids. Triangles! Must move on.