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The demonstration is a process of reasoning in which we draw inferences from results already obtained. These results consist partly of truths established in former propositions, or admitted as obvious in commencing the subject, and partly of truths which follow from the construction that has been made, or which are given in the supposition of the proposition itself. The word hypothesis is used in the same sense as supposition.
To assist the student in following the steps of the reasoning, references are given to the results already obtained which are required in the demonstration. Thus I. 5 indicates that we appeal to the result established in the fifth proposition of the First Book; Constr. is sometimes used as an abbreviation of Construction, and Hyp. as an abbreviation of Hypothesis.
It is usual to place the letters Q.E.F. at the end of the discussion of a problem, and the letters Q.E.D. at the end of the discussion of a theorem. Q.E.F. is an abbreviation for quod erat faciendum, that is, which was to be done; and Q.E.D. is an abbreviation for quod erat demonstrandum, that is, which was to be proved,
1. A POINT is that which bas no parts, or which has no magnitude.
3. The extremities of a line are points.
4. A straight line is that which lies evenly between its extreme points.
5. A superficies is that which has only length and breadth.
6. The extremities of a superficies are lines.
7. A plane superficies is that in which any two points being taken, the straight line between them lies wholly in that superficies.
8. A plane angle is the inclination of two lines to one another in a plane, which meet together, but are not in the same direction.
9. A plane rectilineal angle is the inclination of two straight lines to one another, which meet together, but are not in the same straight line.
Note. When several angles are at one point B, any one of them is expressed by three letters, of which the letter which is at the vertex of the angle, that is, at the point at which the straight lines that contain the angle meet one another, is put between the other two letters, and one of these two letters is somewhere on one of those straight lines, and the other letter on the other straight line. Thus, the angle which is contained by the
straight lines AB, CB is named the angle ABC, or CBA ; the angle which is contained by the straight lines AB, DB is named the angle ABD, or DBA; and the angle which is contained by the straight lines DB, CB is named the angle DBC, or CBD; but if there be only one angle at a point, it may be expressed by a letter placed at that point; as the angle at E.
10. When a straight line standing on another straight line, makes the adjacent angles equal to one another, each of the angles is called a right angle; and the straight line which stands on the other is called a perpendicular to it.
11. An obtuse angle is that which is greater than a right angle.
12. An acute angle is that which is less than a right angle.
13. A term or boundary is the extremity of any thing.
14. A figure is that which is enclosed by one or more boundaries.
15. A circle is a plane figure contained by one line, which is called the circumference, and is such, that all straight lines drawn from a certain point within the figure to the circumference are equal to one another:
16. And this point is called the centre of the circle.
17. A diameter of a circle is a straight line drawn through the centre, and terminated both ways by the circumference.
[A radius of a circle is a straight line drawn from the centre to the circumference.]
18. A semicircle is the figure contained by a diameter and the part of the circumference cut off by the diameter.
19. A segment of a circle is the figure contained by a straight line and the circumference which it cuts off.
20. Rectilineal figures are those which are contained by straight lines :
21. Trilateral figures, or triangles, by three straight
22. Quadrilateral figures by four straight lines:
23. Multilateral figures, or polygons, by more than four straight lines. .
24. Of three-sided figures, : An equilateral triangle is that which has three equal sides :
25. An isosceles triangle is that which has two sides equal :
26. A scalene triangle is that which has three unequal sides :
27. A right-angled triangle is that which has a right angle:
[The side opposite to the right angle in a right-angled triangle is frequently called the hypotenuse.]
28. An obtuse-angled triangle is that which has an obtuse angle :
29. An acute-angled triangle is. that which has three acute angles.
Of four-sided figures,
30. A square is that which has all its sides equal, and all its angles right angles :
31. An oblong is that which has all its angles right angles, but not all its sides equal :
32. A rhombus is that which has all its sides equal, but its angles are not right angles :