Naming angles
Angles have two arms and a vertex.
B is the vertex of the angle, A and C are arms.
The vertex of an angle is always written as the middle letter,
so angle ABC can be written as
or 
Usually, angles are written with a hat on the middle letter, to show the vertex
Angle Types
An acute angle lies between 0° and 90°
A right angle is exactly 90°
An obtuse angle lies between 90° and 180°
A straight angle is 180°
A reflex angle is between 180° and 360°
A full turn is 360°
Lots of pupils mix up an angle's name with its type.
Here is a small mnemonic :
Amy is a cute cat.
What is her name ? Amy
What type of cat is she ? A cute one.
How to use a protractor – measuring angles
- Place protractor so that its centre point is on the vertex and the 0° on the base line is over one of the arms of the angle to be measured.
- Read off from the scale which starts at 0°
- here that is the outside scale, so the angle measures 40°.
Here the inside scale must be used , so the angle measures 130°.
How to use a protractor – drawing angles
- Draw a line.
- Place protractor so that its centre point is on one endpoint of the line and the 0° on the base line is over one of the arms of the angle to be measured.
- Read off from the scale which starts at 0° - here that is the outside scale.
- Mark the paper with a point.
- Remove protractor.
- Join up the new point with the one used as the centre point.
Here the angle drawn measures 40°.
Complementary angles
Supplementary angles
Two complementary angles make a supplementary angle
Vertically opposite angles
When two straight lines cross, the angles on opposite sides of the vertex are called vertically opposite angles.
Corresponding angles
Corresponding angles correspond in position
Corresponding angles are equal on parallel lines.
Alternate angles
Alternate angles lie on either side of the crossing line (traversal) between parallel lines.
The angles in a triangle
The angles in a triangle add up to 180°
Example
© Alexander Forrest