Math Is Fun Forum

question is not much clear. but what I have understood according to that your method is incorrect. I have understood that a triangle(i.e. it's centroid) is situated at a distance R from an axis(which can be anything). & then you are rotating it about the axis. If this is the case then answer depends on the angle that triangle makes with the line joining the axis(point on axis at the same level as centroid) & centorid.
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