Cambridge International Mathematics

Lines, angles and polygons (Chapter 4) 95

Title Figure

Co-interior angles (also

called allied angles)

When two parallel lines are cut by a

third line, then co-interior angles are

supplementary.

a+b= 180

Angles of a triangle The sum of the interior angles of a triangle

is 180 o:

a+b+c= 180

Exterior angle of a triangle The size of the exterior angle of a triangle

is equal to the sum of the interior opposite

angles.

c=a+b

Angles of a quadrilateral The sum of the interior angles of a

quadrilateral is 360 o:

a+b+c+d= 360

Example 1 Self Tutor

Find, giving brief reasons, the value of the unknown in:

ab

a 90 +a+ 40 = 180

) a+ 130 = 180

) a=50

fangles on a lineg

b 2 x¡100 =x

) 2 x¡ 100 ¡x=x¡x

) x¡100 = 0

) x= 100

fequal corresponding anglesg

fsubtractingxfrom both sidesg

fsimplifyingg

a°

b°

c°

a° b°

c°

a°

b°

b° c°

a° d°

DEMO

DEMO

DEMO

x° ()2 ¡-¡100x °

a°

40°

Property

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Y:\HAESE\IGCSE01\IG01_04\095IGCSE01_04.CDR Monday, 15 September 2008 11:50:48 AM PETER