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26. A Scalene Triangle is that whose three sides are all unequal.
27. A Right-angled Triangle is that which has one right-angle.
28. Other triangles are Oblique-angled, and are either Obtuse or Acute.
29 An Obtuse-angled Triangle has one ob. tuse angle.
30. An Acute-angled Triangle has all its three angles acute.
31. A figure of Four sides and angles is called a Quadrangle, or a Quadrilateral.
32 A Parallelogram is a quadrilateral which bas both its pairs of opposite sides parallel. And it takes the following particular names, viz. Rectangle, Square, Rhombus, Rhomboid.
33. A Rectangle is a parallelogram having a right angle.
34 A Square is an equilateral rectangle ; having its length and breadth equal.
35. A Rhomboid is an oblique-angled parallelogram.
36. A Rhombus is an equilateral rhomboid ; having all its sides equal, but its angles oblique.
37. A Trapezium is a quadrilateral which hath not its opposite sides parallel.
38. A Trapezoid has only one pair of opposite sides parallel.
39. A Diagonal is a line joining any two opposite angles of a quadrilateral.
-40. Plane figures that have more than four sides, are, in general, called Polygons: and they receive other particular names, according to the number of their sides or angles, Thus,
41. A Pentagon is a polygon of five sides; a Hexagon, of six sides; a Heptagon, seven ; an Octagon, eight; a Nonagon, nine ; a Decagon, ten; an Undecagon, eleyen ; and a Dodecagon, twelve sides.
42. A Regular Polygon has all its sides and all its angles equal.-If they are not both equal, the polygon is Irregular.
43. An Equilateral Triangle is also a Regular Figure of three sides, and the Square is one of four; the former being also called a Trigon, and the latter a Tetragon.
44. Any figure is equilateral, when all its sides are equal : and it is equiangular when all its angles are equal. When both these are equal, it is a regular figure.
45. A Circle is a plain figure bounded by a curve line, called the Circumference, which is every where equidistant from a certain point within, called its Centre
The circumference itself is often called a circle, and also the Periphery
46. The Radius of a circle is a line drawn from the centre to the circumference.
47. The Diameter of a circle is a line drawn through the centre, and terminating at the circumference on both sides.
48. An Arc of a circle is any part of the circumference.
49. A Chord is a right line joining the extremities of an arc.
50. A Segment is any part of a circle bounded by an arc and its chord.
51. A Semicircle is half the circle, or a seg. ment cut off by a diameter.
The half circumference is sometimes called the Semicircle.
52. A Sector is any part of a circle which is bounded by an arc, and two radii drawn to its extremities.
53. A Quadrant, or Quarter of a circle, is a sector having a quarter of the circumference for its arc, and its two radii are perpendicular to each other. A quarter of the circumference is sometimes called a Quadrant.
54 The Height or Altitude of a figure is a perpendicular let fall from an angle, or its verex, to the opposite side, called the base.
:55. In a right-angled triangle, the side opposite the right angle is called the Hypothenuse ; and the other two sides are called the Legs, and sometimes the Base and Perpendiçular
56. When an angle is denoted by three letters, of which one stands at the angular
E point, and the other two on the two sides, that which stands at the angular point is read in the middle. Thus the angle contained by the lines BA and AD is called the angle B A C BAD or DAB.
57. The circumference of every circle is supposed to be divided into 360 equals parts, called Degrees : and each degree into 60 Minutes, each minutes into 60 Seconds, and so on. Hence a semicircle contains 180 degrees, and a quadrant 90 degrees.
58. The Measure of an angle, is an arc of any circle contained between the two lines wbich form that angle, the angular point being the centre; and is estimated by the number of degrees contained in that arc.
59. Lines, or chords, are said to be Equidistant from the centre of a circle, when perpendiculars drawn to them from the centre are equal.
60. And the right line on which the Greater Perpendicular falls, is said to be farther from the centre.
61. An Angle In a segment is that which is contained by two lines, drawn from any point in the arc of the segment, to the ito extremities of that arc.
62. An Angle On a segment, or an arc, is that which is contained by two lines, drawn from any point in the opposite
or supplemental part of the circumference, to the extremieties of the arc, and containing the arc between them.
63. An angle at the circumference, is that whose angular point is any where in the circum ference And an angle at the centre, is that whese angular point is at the centre.
64. A right-lined figure is Inscribed in a circle, or the circle Circumscribes it, when all the angular points of the figure are in the circumference of the circle.
65. A right-lined figure Circumscribes a circle, or the circle is Inscribed in it, when all the sides of the figure touch the circumference of the circle.
66. Onę right-lined figure is Inscribed in another, or the latter Circumscribes the former, when all the angular points of the former are placed in the sides of the latter.
67. A Secant is a line that cuts a circle, lying partly within, and partly without it.
68. Two triangles, or other right-lined figures, are said to be mutually equilateral, when all the sides of the one are equal to the corresponding sides of the other, each to each : and they are said to be mutually equiangular, when the angles of the one are respectively equal to those of the other.
69. Identical figures, are such as are both mutually equilateral and equiangular ; or that have all the sides and all the angles of the one, respectively equal to all the sides and all the angles of the other, each to each ; so that if the one figure were applied to, or laid upon the other, all the sides of the one would exactly fall upon and cover all the sides of the other; the two becoming as it were but one and the same figure.
70. Similar figures, are those that have all the angles of the one equal to all the angles of the other, each to each, and the sides about the equal angles proportional.
71. The Perimeter of a figure, is the sum of all its sides taken together.
72. A Proposition, is something which is either proposed to be done, or to be demonstrated, and is either a problem or a theorem.
73. A Problem, is something proposed to be done. 74. A Theorem, is something proposed to be demonstrated
75. A Lemma, is something which is premised, or demonstrated, in order to render what follows more easy.
76. A Corollory, is a consequent truth, gained immediately from some preceding truth or demonstration.
77. A Scholium, is a remark or observation made upon something going before it.
If two Triangles have Two Sides and the Included Angle . in the one, equal to Two Sides and the Included Angle in the other, the Triangles will be Identical, or equal in all the side AC be equal to the side DF, then will the two triangles be identical, or equal in all respects. For conceive the triangle ABC to be applied to, or placed
AXIOMS. 1. Things which are equal to the same thing are equal to each other.
2. When equals are added to equals, the wholes are equal. 3. When equals are taken from equals, the remainders are equal.
4. When equals are added to unequals, the wholes are unequal. 5. When equals are taken from unequals, the remainders
6 Things which are double of the same thing, or equal things, are equal to each other.
7. Things which are halves of the same thing, are equal. 8. Every whole is equal to all its parts taken together. 9. Things which coincide, or fill the same space, are identical, or mutually equal in all their parts.
10. All right angles are equal to one another.
А B D E
In the two triangles abc, der, if
and the side BC and the angle c