What is an angle?
An angle is the space between two intersecting lines or surfaces at the point where they meet.
An angle is the space between two intersecting lines or surfaces at the point where they meet.
The size of an angle is measured in degrees. The symbol for degrees is °. A protractor can be used to determine the size of an angle.
You can measure from either end of a protractor, but you need to make sure that you read from the row of numbers (inner or outer) that begins with zero where the line is touching it. You also need to make sure that the point where the two lines meet is directly in the middle.
In the example above, it is the inner row of numbers that we need to look at to measure the size of this angle. This angle measures 135°.
1. Draw a straight line using a ruler. Place the centre of the protractor at the end of the line. Make sure the line passes through the zero on the protractor.
2. Find the angle you want to draw on the protractor, and make a small mark next to it. (Make sure you use the same row of numbers that includes the zero which your straight line is going through.)
3. Use the ruler to draw a straight line between the mark you have made, and the end of the first line you drew. Label the angle.
An acute angle is an angle that is larger than 90º but smaller than 180º.
A right angle is 90º. Right angles are formed when two lines that are perpendicular to each intersect. For example, squares have four right angles.
An obtuse angle is an angle that is larger than 90º but smaller than 180º.
A straight angle is an angle that is 180º. A straight angle is a point on a straight line.
A reflex angle is an angle that is larger than 180º but smaller than 360º.
Angles on a straight line
a + b + c + d = 180°
Angles around a point
e + f + g = 360°
When two straight lines cross,
opposite angles are equal.
The internal angles of a triangle always add up to 180°:
The internal angles of a quadrilateral always add up to 360°:
Here are internal angles of some regular polygons:
If you have enough information about the internal angles of a shape, you can work out the size of a missing angle:
Angles are taught as part of the geometry strands of the maths curriculum. These strands are geometry - position and direction and geometry - properties of shapes.