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be understood. It is seldom requisite to consider
major conjugate angles before Book III.
When the arms of an angle are in the same straight
line, the conjugate angles are equal, and each is then

said to be a straight angle. DEF. 12. When three straight lines are drawn from a point, if

one of them be regarded as lying between the other two, the angles which this one (the mean) makes with the other two (the extremes) are said to be adjacent angles : and the angle between the extremes, through which a line would turn in passing from one extreme through the mean to the other extreme, is

the sum of the two adjacent angles. DEF. 13. The bisector of an angle is the straight line that

divides it into two equal angles. DEF. 14. When one straight line stands upon another straight

line and makes the adjacent angles equal, each of the

angles is called a right angle. OBS. Hence a straight angle is equal to two right angles ;

or, a right angle is half a straight angle. DEF. 15. A perpendicular to a straight line is a straight line

that makes a right angle with it. DEF. 16. An acute angle is that which is less than a right

angle. DEF. 17. An obtuse angle is that which is greater than one right

angle, but less than two right angles. DEF. 18. A reflex angle is a term sometimes used for a major

conjugate angle. DEF. 19. When the sum of two angles is a right angle, each is

called the complement of the other, or is said to be complementary to the other.

DEF. 20. When the sum of two angles is two right angles, each

is called the supplement of the other, or is said to be

supplementary to the other. DEF. 21. The opposite angles made by two straight lines that

intersect are called vertically opposite angles. DEF. 22. A plane rectilineal figure is a portion of a plane surface

inclosed by straight lines. When there are more than three inclosing straight lines the figure is called

a polygon. DEF. 23. A polygon is said to be convex when no one of its

angles is reflex. DEF. 24. A polygon is said to be regular when it is equilateral

and equiangular; that is, when all its sides and angles

are equal. Def. 25. A diagonal is the straight line joining the vertices of

any angles of a polygon. which have not a common

arm.

Def. 26. The perimeter of a rectilineal figure is the sum of its

sides. DEF. 27. The area of a figure is the space inclosed by its

boundary Def. 28. A triangle is a figure contained by three straight

lines. DEF. 29. A quadrilateral is a polygon of four sides, a pentagon

one of five sides, a hexagon one of six sides, and

so on.

GEOMETRICAL AXIOMS.

1. Magnitudes that can be made to coincide are equal. Two straight lines that have two points in common lie wholly

in the same straight line.

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POSTULATES. Let it be granted that A straight line may be drawn from any one point to any other

point. 2. A terminated straight line may be produced to any length in

a straight line. 3. A circle may be described from any centre, with a radius equal

to any finite straight line.

SECTION 1.

ANGLES AT A POINT.

THEOR. I. All right angles are equal to one another. .
CoR. I. At a given point in a given straight line only one

perpendicular can be drawn to that line. COR. 2. The complements of equal angles are equal.

CoR. 3. The supplements of equal angles are equal. THEOR. 2. If a straight line stands upon another straight line, it

makes the adjacent angles together equal to two right

angles. Cor. All the angles made by any number of straight lines

meeting at a point are together equal to four right

angles. THEOR. 3. If the adjacent angles made by one straight line with

two others are together equal to two right angles,

these two straight lines are in one straight line. THEOR. 4. If two straight lines cut one another, the vertically

opposite angles are equal to one another.

SECTION 2.

TRIANGLES. DEF. 30. An isosceles triangle is that which has two sides equal.

DEF. 31. A right-angled triangle is that which has one of its

angles a right angle. An obtuse-angled triangle is that which has one of its angles an obtuse angle. All

other triangles are called acute-angled triangles. DEF. 32. Any side of a triangle may be called the base, and

the opposite angular point is then called the vertex. Def. 33. The side of a right-angled triangle which is opposite

to the right angle is called the hypotenuse. Def. 34. Figures that may be made by superposition to coin

cide with one another are said to be identically equal;

or they are said to be equal in all respects. THEOR. 5. If two triangles have two sides of the one equal to

two sides of the other, each to each, and have likewise the angles included by these sides equal, then the triangles are identically equal, and of the angles those are equal which are opposite to the equal sides.

[By Superposition.]* THEOR. 6. If two triangles have two angles of the one equal to

two angles of the other, each to each, and have likewise the sides between the vertices of these angles equal, then the triangles are identically equal, and of the sides those are equal which are opposite to the

equal angles. [By Superposition.] THEOR. 7. If two sides of a triangle are equal, the angles

opposite to those sides are equal. [By a single

application of Theor. 5, or directly by Superposition.] Cor. If a triangle is equilateral, it is also equiangular. THEOR. 8. If two angles of a triangle are equal, the sides

Throughout this Syllabus a method of proof has been indicated wherever it was felt that this would make the principles upon which the Syllabus is drawn up more readily understood.

opposite to those angles are equal. [By Theor. 6

or directly by Superposition.] COR. If a triangle is equiangular, it is also equilateral. THEOR. 9. If any side of a triangle is produced, the exterior

angle is greater than either of the interior opposite

angles. THEOR. 10. Any two angles of a triangle are together less than

two right angles. COR. 1. If a triangle has one right angle or obtuse angle,

its remaining angles are acute. COR. 2. From a given point outside a given straight line,

only one perpendicular can be drawn to that line. THEOR, 11. If two sides of a triangle are unequal, the greater

side has the greater angle opposite to it. THEOR. 12. If two angles of a triangle are unequal, the greater

angle has the greater side opposite to it. THEOR. 13. Any two sides of a triangle are together greater than

the third side. Cor. The difference of any two sides of a triangle is less

than the third side. THEOR. 14. If from the ends of a side of a triangle two straight

lines are drawn to a point within the triangle, these are together less than the two other sides of the

triangle, but contain a greater angle. THEOR. 15. Of all the straight lines that can be drawn to a

given straight line from a given point outside it, the perpendicular is the shortest; and of the others, those which make equal angles with the perpendicular are equal; and that which makes a greater angle with the perpendicular is greater than that which makes a less angle.

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