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THE

ELEMENTS

O F

E UCL I D.

BOOK I.

A

DEFINITION S.

I.

Point is that which hath no parts, or which hath no mag- See Notes. nitude.

II.

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

V.

A fuperficies is that which hath only length and breadth.

VI.

The extremities of a fuperficies are lines.

VII.

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

VIII.

"A plane angle is the inclination of two lines to one another See N. "in a plane, which meet together, but are not in the fame "direction."

IX.

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

"N. B.

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N. B. When feveral angles are at one point B, any one of them is expreffed by three letters, of which the letter that is at the vertex of the angle, that is, at the point in which the ftraight lines that contain the angle meet one another, is put between the other two letters, and one of these two is fomewhere upon one of those straight lines, and the other upon the other line: Thus the angle which is contained by 'the ftraight lines AB, CB is named the angle ABC, or CBA; that which is contained by AB, DB is named the angle • ABD, or DBA; and that which is contained by DB, CB is called the angle DBC, or CBD; but, if there be only one angle at a point, it may be expreffed by a letter placed at that point; as the angle at E.'

X.

When a ftraight line ftanding on ano

ther straight line makes the adjacent
angles equal to one another, each of
the angles is called a right angle;
and the ftraight line which ftands
on the other is called a perpendicular
to it.

XI.

An obtufe angle is that which is greater than a right angle.

XII.

An acute angle is that which is less than a right angle.

XIII.

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

XIV.

A figure is that which is inclofed by one or more boundaries

XV.

XV.

A circle is a plane figure contained by one line, which is called the circumference, and is fuch that all ftraight lines drawn from a certain point within the figure to the circumference, are equal to one another :

Book T.

XVI.

And this point is called the centre of the circle.

XVII.

A diameter of a circle is a ftraight line drawn through the see Ni centre, and terminated both ways by the circumference.

XVIII.

A femicircle is the figure contained by a diameter and the part of the circumference cut off by the diameter.

XIX.

"A fegment of a circle is the figure contained by a straight "line and the circumference it cuts off."

XX.

Rectilineal figures are thofe which are contained by straight

lines.

XXI.

Trilateral figures, or triangles, by three straight lines.

XXII.

[blocks in formation]

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

XXIV.

Of three fided figures, an equilateral triangle is that which has three equal fides.

XXV.

An ifofceles triangle, is that which has only two fides equal.

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A fcalene triangle, is that which has three unequal fides.

XXVII.

A right angled triangle, is that which has a right angle.
XXVIII.

An obtufe angled triangle, is that which has an obtuse angle.

XXIX.

An acute angled triangle, is that which has three acute anglès.
XXX.

Of four fided figures, a fquare is that which has all its fides
equal, and all its angles right angles.

XXXI.

An oblong, is that which has all its angles right angles, but has not all its fides equal.

XXXII.

A rhombus, is that which has all its fides equal, but its angles are not right angles.

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XXXIII.

See N. A rhomboid, is that which has its oppofite fides equal to one another, but all its fides are not equal, nor its angles right

angles.

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