A 180o Protractor

Outside scale from 0o to 180o going in a clockwise direction.

Inside scale from 0o to 180o going in an

anti-clockwise direction.

You need to be able to measure an angle to the nearest degree. To help you do this you should always:1. Look directly down onto the cross point. (this avoids parallax errors)

2. Always use the protractor in a horizontal manner as shown below.

1

2

Measuring Angles with a

180o Protractor

0 on the outside so use outside

scale

0 on the inside so use inside scale

67o

50o

3

4

Measuring Angles with a

180o Protractor

0 on the outside so use outside

scale

0 on the inside so use inside scale

135o

120o

1

2

Measuring Angles with a

180o Protractor

0 on the outside so use outside

scale

0 on the inside so use inside scale

55o

80o

What’s in the box ?

3

4

Measuring Angles with a

180o Protractor

0 on the outside so use outside

scale

0 on the inside so use inside scale

165o

150o

Measure each angle in the shapes below.

Answer on your worksheet.1 2 3

4 5 6

Measure each angle in the shapes below.

1 2 3

4 5 6

900

900

900

900

1120

1120

680

680670 670

460

730

7301070

1070

900

420

480

1100 1100

1010 1010

1180

Rectangle 3600

Trapezium 3600 Triangle 1800

Parallelogram 3600

Pentagon 5400Triangle 1800

Estimating Angles1 2

3 4

5 6

Naming AnglesAngles are named using the three letters that form them.

The middle of the three letters is the vertex of the angle (pointy bit)

E

D

F

DEF or DÊF or FED or FÊ D

A

B

O

AOB = 27o

A

C

AOC = 122°

Measuring Angles

Types of angle

Acute angle (less than 90°)

Right Angle(90°)

Obtuse angle (between 90°

and 180°)

Straight angle (180°) Reflex angle

(more than 180°)

What’s in the box 2 ?

What is the name of the angle

marked with *?

2 P

R Q S

*

ACB or BCA

What type of angle is this? OBTUSE

1 A

B C D

*

What is the name of the angle

marked with *?

PQS or SQP

What type of angle is this? ACUTE

Complementary angles add up to 90° COSupplementary angles add up to 180° S1 0So the complement of 40° is 50°

The supplement of 40° is 140°

130° x

x = 50°Adjacent angles on a straight line sum to 180°

Angle Reasons

70°x Vertically opposite angles are equalx = 70°

80°110°

120°x

x = 50°

Angles at a point sum to 360°

70°50°

xx = 60°

Angles in a triangle sum to 180°

50° x

x = 50°Base angles of an isosceles triangle are equal

x

x = 60°Each angle in an equilateral triangle is 60°

transversal

110°

x

x = 110°Corresponding angles on parallel lines are equal

50°

x

x = 50°Alternate angles on parallel lines are equal

Angles on Parallel Lines

110°x

x = 70°Co-interior angles on parallel lines sum to 180°

EXTENSION

Types of Triangles (according to sides)Scalene - no equal sidesIsosceles - 2 equal sidesEquilateral - 3 equal sides

Types of Triangles (according to angles)

Acute angled triangle - all angles equalRight angled triangle - one 90° angleObtuse angled triangle - one angle 90°

What’s in the box 3 ?

125o

ao

bo

a =

b =

55

125

100o

35o

co

c =To find c you must use the new fact that in all triangles the three angles add up to 180o 65