Starter
Solve the anagrams
Alerting
?
TRIANGLE
Be Outs
OBTUSE
?
Retail Equal
EQUILATERAL
?
Eco Less Is
?
ISOSCELES
Ace Lens
SCALENE
?
Saturday, 12 April 2025
Angles in triangles
Objectives :
To find missing angles in a triangle
Use an external angle to find missing angles in a triangle
Angles in triangles
Angles in a triangle add to 180°. But why?
The angles form a straight line! Does this work for all triangles?
Angles in triangles
Answers
d
a
b
c
e
g
f
h
i
j
Angles in triangles
This triangle is isosceles. Two of it’s sides and
two of it’s angles are the same. To show which
sides have the same length, we put a line
through them.
a
?
?
b
180 − 38 = 142
𝑏 𝑚𝑢𝑠𝑡 𝑏𝑒 65
65 × 2 = 130
142 ÷ 2 = 71
180 − 130 = 50
Angles in triangles
Be careful with isosceles triangles. The equal angles aren’t all the ones at
the bottom! They’re the ones at the base of the equal lines.
Tell me which angles are equal.
Angles in triangles
Ex. 2 : Find the missing angles.
e
b
c
a
f
g
d
h
o
k
i
l
q
m
n
j
p
s
t
r
Angles in triangles
Consider the following situation
b
a
180 − 130 = 50
a is 50 due to angles on a straight line
50 + 55 = 105
180 − 105 = 75
Angles in triangles
Ex. 3 : Find the missing angles.
d
b
a
f
e
c
h
j
g
i
k
m
o
n
p
q
s
r
Angles in triangles
5 QUICK QUESTIONS CHECK
Now it’s time to check your learning.
You’ll see five questions.
They all have a 45 second timer.
How many can you get right?
Angles in triangles
Question 1
Find the missing angle
Timer
Angles in triangles
Question 2
Find the missing angle
Timer
Angles in triangles
Question 3
Find each missing angle
Timer
Angles in triangles
Question 4
Find each missing angle
Timer
Angles in triangles
Question 5
Find each missing angle
Timer
Angles in triangles
Your answers:
1) 50
2) 74
3) 𝑐 = 58°, 𝑑 = 64°
4) 𝑏𝑜𝑡ℎ 74
5) 𝑔 = 28°, ℎ = 48°
Angles in triangles
Find all the missing angles
Angles in triangles
60