Sec. 1.3 Euclid’sThe Elements 5
- AnOBTUSE ANGLEis that which is greater than a right angle.
- AnACUTE ANGLEis that which is less than a right angle.
- A TERMorBOUNDARYis the extremity of anything.
- AFIGUREis that which is enclosed by one or more boundaries.
- ACIRCLEis a plane figure contained by one line, which is called theCIR-
CUMFERENCE, and is such that all straight lines drawn from a certain point
within the figure to the circumference are equal to one another. - And this point is called theCENTREof the circle.
- ADIAMETERof a circle is a straight line drawn through the centre, and
terminated both ways by the circumference. - ASEMICIRCLEis the figure contained by a diameter and the part of the
circumference cut off by the diameter. - ASEGMENTof a circle is the figure contained by a straight line and the
circumference which cuts it off.
20.RECTILINEAL FIGURESare those which are contained by straight lines.21.TRILATERAL FIGURES,orTRIANGLES, by three straight lines.22.QUADRILATERAL FIGURESby four straight lines.23.MULTILATERAL FIGURES,orPOLYGONS, by more than four
straight lines.- Of three-sided figures, anEQUILATERAL TRIANGLEis that which has
three equal sides. - AnISOSCELES TRIANGLEis that which has two sides equal.
- ASCALENE TRIANGLEis that which has three unequal sides.
- ARIGHT-ANGLED TRIANGLEis that which has a right angle.
- AnOBTUSE-ANGLED TRIANGLEis that which has an obtuse angle.
- An ACUTE-ANGLED TRIANGLEis that which has three acute angles.
- Of four-sided figures, aSQUAREis that which has all its sides equal, and all
its angles right angles. - AnOBLONGis that which has all its angles right angles, but not all its sides
equal.