Cambridge International Mathematics

98 Lines, angles and polygons (Chapter 4)

Atriangleis a polygon which has three sides.

All triangles have the following properties:

² The sum of the interior angles of a triangle is 180 o.

² Any exterior angle is equal to the sum of the interior opposite angles.

² The longest side is opposite the largest angle.

² The triangle is the onlyrigidpolygon.

Discussion #endboxedheading

Example 2 Self Tutor

Find the unknown in the following, giving brief reasons:

ab

a x+ 38 + 19 = 180

) x= 180¡ 38 ¡ 19

) x= 123

fangle sum of a triangleg

b y=39+90

) y= 129

fexterior angle of a triangleg

Example 3 Self Tutor

Find the values of the unknowns, giving brief reasons for your answers.

ab

B TRIANGLES [4.4]

©iStockphoto - Holger Mette

Bridges and other specialised structures often have triangular supports rather than rectangular ones.

The reason for this is that “

”.

the triangle

is the only rigid polygon

1

2

Explain what is meant by “rigid

polygon”?

Why is this important?

38° 19°

x° y° 39°

2 x°

()x¡+¡20° x° 140°

120°

c°

a°

b°

A

B

C

D

E

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Y:\HAESE\IGCSE01\IG01_04\098IGCSE01_04.CDR Friday, 12 September 2008 2:58:20 PM PETER