Measuring and constructing angles with a protractor

Angles of any given size may be drawn, or the size of any angle may be measured, by using an instrument called a pro­tractor (Fig. 13).

A protractor is a semicircular plate graduated from one extremity of the diameter to the other and used to measure angles. The inside numbers show angles in degrees beginning at 0° and going counterclockwise. The outside numbers mark angles in degrees from 0° going clockwise.

Notice that the upper part of the protractor is half a circle. It contains 180 parts. The centre of the semicircle or protractor,is at point 0. Notice that there are two sets of

Fig. 13 Fig. 14

numbers on the curved edge so arranged that one set of numbers begins at the right side of the Protractor and the other begins at the left side. This arrangement permits us to measure angles that open at the left or the right sides.

Problem: Find the number of degrees in angle EFG (Fig. 14)
Method: Place the centre of the protractor at point F so the Point 0 of the protractor is on point F and the zero division line at S on the protractor lies on side FG of the angle. Now read the scale where lines Fe crosses the protractor. Reading from the zero at the right we find that angle EFG contains 30°.

If you were to use¹ the outside scale which starts at zero on the left side of the protractor, the reading would be 150°. Since a 150° - angle would be obtuse, and EFG is acute, you would not have the correct measurement for the angle.

Note:

¹ if you were to use — если бы вы использовали

EXERCISES

I. Read the following words paying attention to the pronunciation:

inside, size, side, acute, obtuse, measuring, constructing, arranging, angle, edge, arrange, circle, semicircle.

II. Make up sentences of your own using the words and expressions given below:

if you were to use, is used to measure, may be drawn, protractor, counterclockwise, permits us to measure, to calculate.

III. Answer the following questions:

1 What is a protractor used for? 2. What do inside numbers on the scale of a protractor show? 3. How many parts does the upper part of the protractor contain? 4. What permits us to measure angles that open at the left or at the right side?

IV. Translate into English:

Для измерения углов употребляется специальный прибор- транспортир. Этот прибор представляет собой полукруг, дуга которого разделена на 180 частей. Чтобы измерить угол, накладывают транспортир так, чтобы Центр транспортира совпал с вершиной угла, а радиус был расположен на одной стороне угла. Тогда число градусов, содержащихся Луге заключенной между сторонами угла, покажет нужную величину. При помощи транспортира можно также начертить угол, содержащий данное число градусов.

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