# Page:The Elements of Euclid for the Use of Schools and Colleges - 1872.djvu/27

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DEFINITIONS.
1. A term or boundary is the extremity of any thing.
2. A figure is that which is enclosed by one or more boundaries.
3. A circle is a plane figure contained by one line, which is called the circumference, and is such, that all straight lines drawn from a certain point within the figure to the circumference are equal to one another:
4. And this point is called the centre of the circle.
5. A diameter of a circle is a straight line drawn through the centre, and terminated both ways by the circumference.

[A radius of a circle is a straight line drawn from the centre to the circumference.]

6. A semicircle is the figure contained by a diameter and the part of the circumference cut off by the diameter.
7. A segment of a circle is the figure contained by a straight line and the circumference which it cuts off.
8. Rectilineal figures are those which are contained by straight lines:
9. Trilateral figures, or triangles, by three straight lines:
10. Quadrilateral figures by four straight lines:
11. Multilateral figures, or polygons, by more than four straight lines.
12. Of three-sided figures,

An equilateral triangle is that which has three equal sides:

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