Articoli correlati a A primary geometry; with simple and practical examples...
A primary geometry; with simple and practical examples in plane and projection drawing, and suited to all beginners - Brossura
Sinossi
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1887 Excerpt: ...between them (obtuse in (5) and acute in (6)) on the size of the circle containing the given points? 193. Having now considered the two extreme regular polygons, the equilateral triangle, and the circle, we will take up such other regular polygons as are most important, beginning with Fig. 95. Fig. 90, THE SQUARE. The square, Fig. 95, is a figure of four equal sides and four equal angles. These are right angles, because a person, starting at A and facing B, and walking round the square to the starting-point, would have turned entirely around once; and because, in turning equally at four different points, B, C, D, and A, he would have turned a quarter round, or one right angle, at each point. 194. If we pin together three thin strips of wood or card-board so as to form a triangle, c as in Fig. 96, it will be found that they will form a rigid or inflexible frame; that is, one whose shape cannot be altered, or, in still other words, a figure of invariable form. But if we pin together four equal strips to form a square, we shall find that the corners or joints will be flexible; that is, a square is not a rigid figure, and the same is true of all figures, except the triangle. 195. The lines, as AD and BC, Fig. 97, which join opposite corners of a square are called its diagonals. The square has two diagonals. Each diagonal divides the square into two triangles, as ABC and BCD, which are evidently, 1st, equal; 2d, rightangled; 3d, isosceles. Also, each of the two angles at A, B, C, and D, being evidently half a right angle, is an angle of 45. 196. By examining the square in connection with the idea of symmetry (143), we see that it has four axes of symmetry, or centre lines, in two pairs: one pair, AD and BC, containing opposite corners; the other, ab and np, the ...
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