Definition of "decangle"

And from hence also shall be the geodesy of the Icosaedrum. For the finding out of the heighth of the pyramis, there is the semidiagony of the side of the decangle and the halfe ray of the circle: But the side of the decangle is a right line subtending the halfe periphery of the side of the quinquangle, or else the greater segment of the ray proportionally cut.

1636, Peter Ramus, translated by Peter Bedwell, The Way To Geometry: [Being Necessary and Usefull for Astronomers, Enginees, Geographers,. Architects, Land-meaters, Carpenters, Sea-men & Etc.]‎, pages 277–278

The ſide of a Hexagon or Radius being given, the ſide of a Decangle or ſubtenſe of 30 deg. is alſo given ; for by the 19 of the ſecond, the Semi-radius being deducted from the ſquare root of the ſquares of Radius and the half Radius added together, the remainer is the ſide of a Decangle.

1658, John Newton, Trigonometria Britanica‎, page 9

Subtract the ſide of a decangle from the diameter, the remainer multiplied by the radius ,ſhall be the ſquare of one ſide of a Pentagon, whoſe ſquare root ſhall be the ſide itſelf, or ſubtense of 72 degrees.

1671, John Newton, Institutio mathematica: or, A mathematical institution‎, page 54