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THE

ELEMENTS OF PLANE GEOMETRY;

OR,

THE FIRST SIX BOOKS

Or

EUCLID. .

ROM THE TEXT OF ROBERT SIMSON, M.D.,

Emeritus Professor of Mathematics in the University of Glasgow.

EDITED BY

WILLIAM DAVIS, B.A.

FIRST THOUSAND.

LONDON: LONGMAN, GREEN, LONGMAN, ROBERTS, AND GREEN.

EDINBURGH: OLIVER AND BOYD. DUBLIN: J, ROBERTSON AND CO.

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LONDON : PRINTED BY TAYLOR AND GREENING, GRAYSTOKE PLACE,

FETTER LANE, HOLBORN, E.C.

EUCLID'S

ELEMENTS OF PLANE GEOMETRY.

BOOK 1.

DEFINITIONS.

I.
A POINT is that which has no parts, or which has no magnitude.

II.
A line is length without breadth.

III. The extremities of a line are points.

IV.
A straight line is that which lies evenly between its extreme points.

V.
A superficies is that which has only length and breadth.

VI.
The extremities of a superficies are lines.

VII. A plane superficies is that in which any two points being taken, the straight line between them lies wholly in that superficies.

VIII. A plane angle is the inclination of two lines to one another in a plane, which meet together, but are not in the same direction.

IX. A plane rectilineal angle is the inclination of two straight lines to one another, which meet together, but are not in the same straight line.

N.B. - When several angles are at one point B, any one of them is expressed by three letters, of which the letter that is at the vertex of tho angle, that is, at the point in which

А the straight lines that contain the angle meet one another, is put between the other two letters, and one of these two is somewhere upon one of those straight lines, and the other upon the other straight line: B tbus the angle which is contained by the straight lines, A B, CB, is named the angle A B C, or CBA; that which is contained by A B, D B, is named the angle A BD, or D BA; and that

D

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which is contained by D B, CB, is called the angle D B C, or CBD; but, if there be only one angle at a point, it may be expressed by a letter placed at that point; as the angle at E.

X.
When a straight line standing on another straight line makes
the adjacent angles equal to one another, each of these angles
is called a right angle; and the straight line which stands on
ibe other is called a perpendicular to it.

XI.
An obtuse angle is that which is greater than a right angle.

XII.
An acute angle is that which is less than a right angle.
Both obtuse angles and acute angles are termed oblique.

XIII.
A term or boundary is the extremity of any thing.

XIV.
A figure is that which is enclosed by one or more boundaries.

XV.
A circle is a plano 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.

XVI.
And this point is called the centre of the circle.

XVII. A diameter of a circle is a straight line drawn through the centre, and terminated both ways by the circumference.

XVIII. A semicircle is the figure contained by a diameter and the part of tho circumference cut off by the diameter.

XIX. A segment of a circle is the figure contained by a straight line, and the part of the circumference it cuts off.

XX.
Rectilineal figures are those which are contained by straight lines.

XXI.
Trilateral figures, or triangles, by three straight lines.

XXII.
Quadrilateral by four straight lines.

XXIII. Multilateral figures, or polygons, by more than four straight lines.

XXIV. Of three-sided figures, an equilateral triangle is that which has three equal sides.

A

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