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From the results of Exploration Activity 3, it is found that for a triangle,
(i) the sum of all the interior angles is 180°.
(ii) the sum of an interior angle and its adjacent exterior angle is 180°.
(iii) an exterior angle is the sum of two opposite interior angles.

a d a + b + c = 180°
c c + d = 180°
b d = a + b The angle on a straight
line is 180°.

3

Find the value of x in each of the following diagrams.
(a)
92° 79°
(b) 28°
x
37° x

The sum of all The exterior
(a) 92° + 37° + x = 180° the interior (b) x = 79° + 28° angle is the sum
129° + x = 180° angles is 180°. = 107° of two opposite
x = 180° – 129° interior angles.
= 51° CHAPTER

9

Self Practice 9.2b

1. Find the value of p in each of the following triangles.
(a) (b) (c) (d)
p
100° 126° 96°
p p p
45° 43° p

2. Find the value of x in each of the following diagrams.
(a) (b) (c) (d)
x x
66°
x

x x 144° 44° 38°
152°

209
Basic Polygons

09 TB Math F1.indd 209 11/10/16 12:18 PM