Cambridge International Mathematics

Lines, angles and polygons (Chapter 4) 99

a 2 x+x+(x+ 20) = 180

) 4 x+ 20 = 180

) 4 x= 160

) x=40

fangles of a triangleg

b a= 180¡140 = 40

Likewise b= 180¡120 = 60

But a+b+c= 180

) 40 + 60 +c= 180

) 100 +c= 180

) c=80

fangles on a lineg

fangles of a triangleg

EXERCISE 4B

1 Find the unknown in the following, giving brief reasons:

abc

def

2 The following triangles arenotdrawn to scale. State the longest side of each triangle.

abc

de f

gh i

95°

23°

a° 46°

25°

c°

42° 47°

b°

25 °

f° 33 °

46 °

e°

47°

81°

d°

75°

A 20° 85°

B

C 52°

103°

A

B

C

24°

78°

C

B

A

38°

17°

A

B

C

32°

32°

C

A

B

7°

120°

B

C

A

90°

A

C

B

14°

79°

A

C

B

11° 77°

A

C

B

The longest side

is opposite the

largest angle.

IGCSE01

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Y:\HAESE\IGCSE01\IG01_04\099IGCSE01_04.CDR Monday, 15 September 2008 11:55:33 AM PETER