This section of revision notes will illustrate facts about angles between lines and in polygons.
Basic Angles Facts
Any two angles that add up to 180 degrees are known as supplementary angles.
Angles on a Straight Line
Angles on a straight line add up to 180 degrees.
Angles at a Point
The angles at a point add up to 360 degrees.
Angles in polygons
A polygon is a many sided shape the simplest being a triangle with just three sides. A polygon has interior angles which are angles inside the shape and exterior angles which are angles created outside the shape by extending their sides (see the diagram below).
Interior Angles in Triangles
The interior angles in a triangle add up to 180 degrees.
Interior Angles in Quadrilaterals
The angles in a quadrilateral add up to 360 degrees.
Interior angles in any polygon
The interior angles in an n-sided polygon will add up to 180(n - 2) degrees.
For example, the pentagon below the interior angles will add up to 180(5 - 2) = 180 x 3 = 540 degrees.
In regular polygon (where all the sides are of equal length), all the interior angles are equal. We can find an interior angle of a regular polygon by finding the total they all add up to using the above method and then dividing that total by the number of sides.
For example, the interior angles in a regular pentagon will add up to 540 degrees. There are 5 sides to a pentagon. Therefore, one interior angle is 540 divided by 5, which gives 108 degrees.
Exterior angles in any polygon
The exterior angles of any polygon add up to 360 degrees.
The diagram shows the exterior angles of a pentagon, which will add up to 360 degrees.
In a regular polygon, all exterior angles are equal.
For example one exterior angle of a regular pentagon is 360 divided by 5, which gives 72 degrees.
The Interior And Exterior Angle At Any Corner
At any corner of a polygon, the interior and exterior angles are supplementary. That is, they add up to 180 degrees.
- Some angle questions will ask for reasons for your answers. For these questions you need to say which rule you used to work out the answer (such as angles in a triangle add up to 180 degrees or the angles are alternate).
- Do not try to measure angles in questions as the diagrams will not be drawn accurately.
- There may be more than one way to work out some questions. If you are stuck doing the question one way see if you can find another way to answer it.
- You may have to find some unlabelled angles first in order to work out the labelled ones the question asks for.