**Angles in Triangles**

Starter

Find the missing angles:

**L.O. - To demonstrate the sum of angles in a triangle and calculate missing angles in triangle**

57

o

a

145

o

155

o

b

b

57

a

b

o

Starter

Find the missing angles:

57

o

123

145

o

155

o

30

30

57

57

o

o

o

o

o

Main Activity

Complete the Angles in triangles sheet

Example

**Key**

Examples

Examples

**Activities**

**Activity**

Answers

Answers

c

123

123

o

o

Angles in triangles

Cut out a triangle. Mark the angle and put a dot in each corner and tear off the corners.

Can you arrange the three pieces so that that dots are together? What shape do you get?

Angles in triangles

Cut out a triangle. Put a dot in each corner and tear off the corners.

The three pieces form a straight line.

Angles on a straight line add up to 180

o

68

62

o

o

Find the value of angle ABC

A

B

C

P

Q

R

42

o

Find the values of the angles QPR and PRQ.

Example

68

62

o

o

Find the value of angle ABC

A

B

C

P

Q

R

42

o

Find the values of the angles QPR and PRQ.

62 + 68 + ABC = 180

130 + ABC = 180

ABC = 180 - 130 = 50

o

o

o

o

o

o

o

o

QPR = 42

42 + 42 + PRQ = 180

84 + PRQ = 180

PRQ = 180 - 84 = 96

o

o

o

o

o

o

o

o

o

**Worked**

Example

Example

**Worked**

Example

Example

Main Activity

Complete the Angles in triangles sheet

a = 60 b = 50 c = 45 d = 70

e = 43 f = 43 g = 35 h = 46

i = 60 j = 39 k = 45 l = 71

o

o

o

o

o

o

o

o

o

o

o

o

Plenary

Plenary

45 + 55 + ACB = 180

100 + ACB = 180

ACB = 80

x + ACB = 180

x + 80 = 180

x = 180 - 80 = 100

o

o

o

o

o

o

o

o

o

o

o

o

Main Activity

The large triangle is equilateral. All other triangles are isosceles.

How many angles can you work out in the picture?