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a regular star
A 'concave decagon' is a ten-sided shape with every other corner pushed in towards the centre. It forms a regular five-pointed star.
There is no such shape. A 2d shape with 5 sides and all equal angles must be a regular pentagon (or a 5-pointed star if you stretch the definition of "all angles"). And a regular pentagon - or star - has 5 lines of symmetry.
A regular pentagon if your talking external walls, internal lines is a 5 pointed star.
yes an eclipse is a regular shape