Page 170 - ICSE Math 6

Use of protractor to measure an angle Outer scale Inner scale
100
A protractor is a geometrical instrument which is used 60 70 80 90 80 110 120
for measuring and constructing angles. On the circular part 50 70 60 50 130
40
of the protractor, angles are marked in degrees from 0° to 30 170 160 150 140 130 120 110 100 40 140 150
180°. On the inner scale, angles are marked in anticlockwise 20 30 160
direction and on the outer scale, angles are marked in 10 20 170
10
clockwise direction. 0 180 0 180
Mid-point Base line

A

Suppose, we want to find the magnitude of ∠AOB given in
the adjoining figure. To do so, follow the steps given below.
Step 1: Place the protractor so that its mid-point
coincides with vertex O and the base line is on
ray OB. O B

Step 2: Locate the angle marked along ray OA A
on the protractor. As the 0° mark of the 60 70 80 90 100 110 120
80
inner scale falls on ray OB, therefore we 50 70 60 50 130
40
will consider this scale for measuring the 30 170 160 150 140 130 120 110 100 40 140 150
angle. Thus, ∠AOB = 50°. 20 30 20 160

10 10 170
0 180 O 0 180 B
Points to remember
• Angles opening on the right are measured using the inner scale.
• Angles opening on the left are measured using the outer scale.

Example 8: Measure the following angles using a protractor.

A
P P
(a) (b) (c)
Q Q R
R
O B

Solution: (a) Place the protractor so that its mid- A
point coincides with O and the base
line coincides with ray OB. Locate 70 80 90 100 110
the degree marked along ray OA 50 60 80 70 60 120 130
on the inner scale of the protractor. 40 50 40 140
Thus, ∠AOB = 100°. 30 170 160 150 140 130 120 110 100 30 150
20 20 160
10 10 170
0 180 O 0 180 B

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