Unit 5 Section 6 : Finding Angles in Triangles
Sum of the Interior Angles of a Triangle
The angles inside a triangle are called interior angles.
The diagram below shows the interior and exterior angles of a triangle.
The three interior angles in a triangle will always add up to 180°.
At each corner the exterior and interior angles are on a straight line, so at each corner these two angles add up to 180°.
Types of Triangle
When looking at the sides, there are three types of triangle: Isosceles, Equilateral and Scalene
![](s6egisos.gif) |
Isosceles
An isosceles triangle has two lengths which are the same as each other, and two angles which are the same as each other.
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![](s6egequi.gif) |
Equilateral
An equilateral triangle has three equal lengths, and all the angles are equal which means they are each 60°.
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![](s6egscal.gif) |
Scalene
A scalene triangle has three different length sides, and three different angles.
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Example Question
Work out the answer to the question then click on the button marked
to see whether you are correct.
Look at the triangle in the diagram below.
What is the size of missing angle a?
![](showme417.gif)
 
Exercises
Work out the answers to the questions below and fill in the boxes. Click on the
button to find out whether you have answered correctly. If you are right
then will appear and you should move on to the next
question. If appears then your answer is wrong. Click
on to clear your original answer and have another go.
If you can't work out the right answer then click on
to see
the answer.
You have now completed Unit 5 Section 6
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