Page 171 - ICSE Math 6

(b) Place the protractor so that its mid-
point coincides with O and the base P 70 80 90 100 110
120
line coincides with ray QR. The 0° 50 60 80 70 60 130
mark of the outer scale is along ray 40 50 40 140
QR, therefore we will consider the 30 170 160 150 140 130 120 110 100 30 150
outer scale for measuring the given 20 20 160
angle. Locate the degree marked 10 10 170
along ray PQ on the protractor. R 0 180 Q 0 180
Thus, ∠PQR = 44°.

P
(c) The arc representing the angle is
drawn on the outer side, therefore
the given angle is a reflex angle.
So, first measure ∠PQR as shown Q R
in the adjoining figure.
Thus, ∠PQR = 30°. 50 60 70 80 90 100 70 60 120 130
110
80
40
30 170 160 150 140 130 120 110 100 50 40 140 150 P
30
As the sum of ∠PQR and the given 20 20 160
reflex angle is 360°, therefore the 10 10 170
required angle is 360° – 30° = 330°. 0 180 Q 0 180 R

Use of protractor to construct an angle
Suppose we want to construct an angle of 36°. To do so, follow the steps given below.

Step 1: Draw a ray, say OP, with initial point O. O P

Step 2: Place the protractor on ray OP such that its 100
mid-point coincides with point O and the 60 70 80 90 80 110 120
base line coincides with ray OP. 40 50 70 60 50 130 140 Q

30 170 160 150 140 130 120 110 100 40 150
Step 3: The 0° mark of the inner scale falls on 20 30 160
ray OP. Therefore, we will consider the 10 20 170
10
inner scale for constructing the angle. 0 180 0 180
Mark a point, say Q, on the division which O P
represents 36°.
Q
Step 4: Remove the protractor and draw a ray
joining the points O and Q. Thus, ∠POQ
is the required angle.

O P

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