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be understood. It is seldom requisite to consider
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 tlırough 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.
A SYLLABUS OF
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
I. Magnitudes that can be made to coincide are equal.
in the same straight line. 3. A finite straight line has one and only one point of bisection. 4. An angle has one and only one bisector.
Let it be granted that
a straight line. 3. A circle may be described from any centre, with a radius equal
to any finite straight line.
ANGLES AT A POINT.
THEOR. I. All right angles are equal to one another.
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. 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.
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
A SYLLABUS OF
which has one of its angles an obtuse angle. All
other triangles are called acute-angled triangles. DEF. 32. A triangle is sometimes regarded as standing on a
selected side which is then called its base, and the intersection of the other two sides is called the vertex. When two of the sides of a triangle have been mentioned, the remaining side is often called
the base. 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; and every part of one being equal to a corresponding part of the other, 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 contained 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. The angles at the base of an isosceles triangle are
equal to one another. [By a single application of
Theor. 5, or directly by Superposition.] Cor. If a triangle is equilateral, it is also equiangular. THEOR. 7. If two triangles have two angles of the one equal to
two angles of the other, each to each, and have likewise the arms common to these angles equal,
* 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.
then the triangles are identically equal, and of the sides those are equal which are opposite to the
equal angles. [By Superposition.] THEOR. 8. If the angles at the base of a triangle are equal to
one another, the triangle is isosceles. [By Theor. 7,
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. The greater side of every triangle has the greater
angle opposite to it. THEOR. II. The greater angle of every triangle has the greater
side opposite to it THEOR. I 2. 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. 13. If from the ends of the 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. 14. If two triangles have two sides of the one equal
to two sides of the other, each to each, but the included angles unequal, then the bases are unequal, the base of that which has the greater angle being
greater than the base of the other. THEOR. 15. If two triangles have the three sides of the one equal
to the three sides of the other, each to each, then the triangles are identically equal, and of the angles those are equal which are opposite to equal sides.