1. Properties of Triangles
Objectives:
G Grade Identify isosceles, equilateral and right-angled
triangles.
Use the word ‘congruent’ when triangles are
identical.
E Grade Show that the angles of a triangle add up to 180o
and use this to find angles.
Show that the exterior angle of a triangle is equal
to the sum of the interior opposite angles.
Use angle properties of isosceles, equilateral
and right-angled triangles.

2. Properties of Triangles The external angle = a + cb c A B C
When a side of a polygon is extended an exterior angle is created
We know that the sum of internal angles (a + b + c) = 180o
We also know that angles on a line add up to 180o
Therefore if: a + c + b = 180o and b + external angle = 180o
The external angle = a + c

3. Properties of Triangles
Example
39o
81o y x x y
x is an exterior angle so
x = = 120o
39o
81o x
x is again an exterior angle so
120 = y + 90
so y = 30o

4. Properties of Triangles
Now do these:
c = = 137o
d = = 113o
e = = 106o
a = = 82o
b = = 90o
132 = x + 90
x = 42o

5. Properties of Triangles
Now do these:
r = = 108o
= 40o
p = = 112o
= 62o
q = = 125o
s = 180 – ( )
= 78o
Bearing
=311o or
=311o
x = = 41o

6. Properties of Triangles
Worksheet