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