AN INDEX

SHOWING THE PROPOSITIONS OF LEGENDRE WHICH CORRESPOND TO THE PRINCIPAL PROPOSITIONS OF THE FIRST SIX BOOKS OF EUCLID.

ELEMENTS OF GEOMETRY.

BOOK I.

THE PRINCIPLES.

Definitions.

1. GEOMETRY is the science which has for its object the measurement of extension.

Extension has three dimensions, length, breadth, and height, or thickness.

2. A line is length without breadth, or thickness.

The extremities of a line are called points: a point, therefore, has neither length, breadth, nor thickness, but position only.

3. A straight line is the shortest distance from one point to another.

4. Every line which is not straight, or composed of straight lines, is a curved line.

Thus, AB is a straight line; ACDB is a broken line, or one composed of straight A lines; and AEB is a curved line.

The word line, when used alone, will designate a straight line; and the word curve, a curved line.

5. A surface is that which has length and breadth, without height or thickness.

6. A plane is a surface, in which, if two points be assumed at pleasure, and connected by a straight line, that line will lie wholly in the surface.

7. Every surface, which is not a plane surface, or composed of plane surfaces, is a curved surface.

8. A solid or body is that which has length, breadth, and thickness; and therefore combines the three dimensions of extension.

9. When two straight lines, AB, AC, meet each other, their inclination or opening is called an angle, which is greater or less as the lines are more or less inclined or opened. The point of intersection A is the vertex of the angle, and the lines AB, AC, are its sides.

The angle is sometimes designated simply by the letter at the vertex A; sometimes by the three letters BAC, or CAB,. the letter at the vertex being always placed in the middle. Angles, like all other quantities, are susceptible of addition, subtraction, multiplication, and division.

Thus the angle DCE is the sum of the two angles DCB, BCE; and the angle DCB is the difference of the two A angles DCE, BCE.

10. When a straight line AB meets another straight line CD, so as to make the adjacent angles BAC, BAD, equal to each other, each of these angles is called a right angle; and the line AB is said to be perpendicular to CD.

11. Every angle BAC, less than a D right angle, is an acute angle; and every angle DEF, greater than a right angle, is an obtuse angle.

12. Two lines are said to be parallel, when being situated in the same plane, they cannot meet, how far soever, either way, both of them be produced.

13. A plane figure is a plane terminated on all sides by lines, either straight or curved.

If the lines are straight, the space they enclose is called a rectilineal figure, or polygon, and the lines themselves, taken together, form the contour, or perimeter of the polygon.

14. The polygon of three sides, the simplest of all, is called a triangle; that of four sides, a quadrilateral; that of five, a pentagon; that of six, a hexagon; that of seven, a heptagon; that of eight, an octagon; that of nine, a nonagon; that of ten, a decagon; and that of twelve, a dodecagon.

C

15. An equilateral triangle is one which has its three sides equal; an isosceles triangle, one which has two of its sides. equal; a scalene triangle, one which has its three sides unequal. 16. A right-angled triangle is one which has a right angle. The side opposite the right angle is called the hypothenuse. Thus, in the triangle ABC, right-angled at A, the side BC is the hypothenuse.

B

17. Among the quadrilaterals, we distinguish :

The square, which has its sides equal, and its angles right-angles.

The rectangle, which has its angles right angles, without having its sides equal.

The parallelogram, or rhomboid, which has its opposite sides parallel.

A

The rhombus, or lozenge, which has its sides equal, without having its angles right angles.

And lastly, the trapezoid, only two of whose sides are parallel.

18. A diagonal is a line which joins the vertices of two angles not adjacent to each other. Thus, AF, AE, AD, AC, are diagonals.

B

E

G

19. An equilateral polygon is one which has all its sides. equal; an equiangular polygon, one which has all its angles equal.

20. Two polygons are mutually equilateral, when they have their sides equal each to each, and placed in the same order;