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The size of an angle can be estimated by recognising the right angle first. An angle that
appears more than a right angle has an angle greater than 90°. An angle that appears less
than a right angle has an angle less than 90°. For instance,

Right angle = 90° An angle less than 90° An angle greater than 90°
P

∠PQR appears slightly less than a right angle.
Thus, ∠PQR can be estimated at about 80°.
Q R

From what you have learnt, the size of A
80 100
an angle can be measured more accurately 60 70 90 80 70 110 120
50
using a protractor. 40 140 130 120 110 100 60 50 130 140
30 40 150
150 30
Size of ∠ABC is 10 20 160 20 160
exactly 135°. 0 170 180 10 0 170 180 C
B
Self Practice 8.1b

1. Estimate the length of each of the following line segments. Explain how you would
do the estimation.
CHAPTER
8 (a) (b) Q
A

B P
2. Measure each of the following line segments.
(a) (b)
A Q
B P

3. Estimate the size of each of the following angles. Explain how you would do the
estimation.
(a) (b) (c)

4. Using a protractor, measure each of the following angles accurately:
(a) (b) (c)

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Chapter 8

08 TB Math F1.indd 172 11/10/16 12:17 PM