An edge is the curve formed where two of the planes defining [bordering] the 3D shape meet. It is often a straight line where the planes defining the shape meet but, as in the case of a hemisphere, an edge could be a circular boundary where the flat face meets the curved face.
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The special features of 3D shapes are they all are 3D which means they are fat but 2D shapes are flat they can not stand up.
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Polygons are 2d shapes whereas polyhedrons are 3d shapes
3D shapes have three dimensions, namely length, breadth and height.
No because 2d shapes are plane such as polygons but example of 3d shapes are: pyramid, cone, cuboid, cylinder, sphere ... etc