UNIT 8 SEC 2.1 Triangles
29
April 2016
20:00
The interior angles of
a triangle are the angles formed inside the triangle
The angles in a triangle add up to 180◦.
First, a key step was experimenting with
the triangle, spotting a pattern
An addition to a geometric diagram, in
order to help prove a fact about the
Once you have come up with the main ideas
of a proof, the next stage is to
Setting out a
geometric argument
1. State the general fact that
is to be proved, including any given
2. Draw a diagram that contains
the information that is given,
3. Add any useful
constructions.
4. Proceed step by step from
what is given to what is to be proved,
Example 3 Proving
the result about the angles in a triangle
Prove that the angles of
any triangle add up to 180◦.
Solution
State the general fact to
be proved.
Here we prove that the
angles in a triangle add up to 180◦.
Draw a suitably-labelled
diagram containing the information that you
know.
Consider -ABC.
Add any necessary
constructions.
Draw a line through B, parallel to the side
AC.
Proceed step by step from
what you know to the fact that is to be
∠DBA and ∠BAC are
alternate angles. Therefore ∠DBA = θ.
∠EBC and ∠BCA are
alternate angles. Therefore ∠EBC = φ.
∠DBA, ∠ABC and ∠EBC are on the
straight line passing through B and
So θ + ψ + φ = 180◦.
But this is the sum of the
angles in -ABC.
Thus the angles in a
triangle add up to 180.
You will be looking at some more proofs
later in the unit, but in the rest of
‘eye -sos-eh-lees’.
If a triangle has all its sides the same
length, then all its angles are equal
Each angle of an equilateral triangle is
60◦.
If a triangle has just two
sides that are the same length, then it also has
A triangle in which one angle is equal to
90◦ is
called a right-angled
Example 4 Finding
angles in an isosceles triangle
Calculate the base angles
in an isosceles right-angled triangle, such as the
Solution
Start from what you know.
Let each base angle of the
triangle be θ (the same letter can be used for
90◦, θ and θ.
Using the fact that the
angle sum of a triangle is 180◦ gives
the equation
90
◦
+ 2θ = 180
Solve the equation.
Rearranging the equation
gives
2θ = 180
◦− 90
2θ = 90
θ = 45
So each base angle of the
triangle is 45◦.
If you can show that two angles in a
triangle are equal to each other, then
A triangle that is neither equilateral
nor isosceles is known as a scalene
The next example involves putting
together several results that you have
Example 5 Finding
angles related to a garden shed
The diagram below shows the
front of a garden shed, including a wooden
20
Solution
Use the angle properties of
straight lines, parallel lines and angles.
Since the angles on a
straight line add up to 180◦,
γ = 180
◦− 65
= 115
Since the lines DC and AB are parallel, the
angles marked β and 65◦ are
corresponding angles. So
β = 65
Draw construction lines to
help you to find the unknown angles.
Since the lines AE and BC are parallel, ∠FAE and ∠ABC are
corresponding angles. So ∠FAE = β = 65◦.
Since the angles on a
straight line add up to 180◦,
α = 180
◦− 65
= 115
Since the angles in -CGD add up to
180◦,
∠CDG = 180
◦− 90
◦− 65
= 25
Hence, since the angles on
a straight line add up to 180◦,
δ = 180
◦− 25
= 155
Example 6 Finding an
expression for an angle
Find an expression for ∠ABC in the
triangle below in terms of α.
Solution
Since the sum of the angles
in a triangle is 180◦,
∠ABC + 50
+ α = 180
Hence
∠ABC = 180
◦− 50
◦− α
so
∠ABC = 130
◦− α.
∠ABC can now be labelled as 130◦− α on the diagram.
Next, you are asked to use the dynamic
geometry resource to discover
something about the sum of the exterior
angles of a triangle. An exterior
angle of a triangle is
the angle formed outside the triangle by one side and
an extension of the adjacent
side,