Practical arithmetic; an introduction to elementary mathematics for scholars between the ages of 9 and 12 by Consterdine A & Andrew S. O. (Samuel Ogden) 1868-1952

Author:Consterdine, A & Andrew, S. O. (Samuel Ogden), 1868-1952
Language: eng
Format: epub
Tags: Arithmetic
Publisher: London : J. Murray
Published: 1905-03-25T05:00:00+00:00

FIG. 38.

Each pair of arms ought to be in the same straight line. Test with the edge of your ruler whether they are in the same straight line. If they are not, the angles are not right angles ; if they are, the angles are right angles.

258. Take your compasses once more. Open the

FIG. 39.

arms until they form a right angle. Then open them further until they form * an angle equal to two right

angles. The two arms will then be in a straight line ; and the angle formed by them is called a straight angle. (Fig. 39.)

Test of a Bight Angle

259. Whenever the outer arms of two equal adjacent angles lie in the same straight line or, in other words, form a straight angle, each of the two equal angles is a right angle.

Use this test in order to discover other right angles on your models.

Take a piece of paper with a straight edge, and make a right angle by folding.

260. If you continued moving one leg of your compasses until it had turned right round and met the other leg again, it would have turned through all the space round a point. How many straight angles would it have turned through ?

Since two right angles make a straight angle, four right angles ought to make two straight angles, i.e., they ought to occupy all the space round a point. Test whether they do so, by placing right angles on four cubes together at a point.

Make four right angles round a point by folding and refolding a piece of paper.

261. Which angles on your set-squares do you think are right angles ? Test them in the following way.

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