Does a decagon tesselate

Answer 1

yes of course

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A decagon does NOT tessellate. All triangles and quadrilaterals do, there are 14 tessellating pentagons and a number of hexagons (including regular hexagons). There are no tessellations which use polygons of the same shape - regular or irregular - for polygons with 7 or more sides.

Answer 2

A decagon only tessalates if it is a regular decagon (all of the angles are the same)

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No. Even if it is regular, on its own it cannot tessellate.