A ten-sided polygon. In Flatland, a decagon would be one of the higher-skilled professionals, such as a doctor.

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A polygon of ten sides, having an internal angle sum of 1440 degrees.

For a regular decagon with sides of length x,

Area = x27.694

Dec"a*gon (?), n. [Pref. deca- + Gr. a corner or angle: cf. F. d'ecagone.] Geom.

A plane figure having ten sides and ten angles; any figure having ten angles. A regular decagon is one that has all its sides and angles equal.

 

© Webster 1913.

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