An angle is formed where two straight lines meet. We measure them in degrees and there are 360 degrees in a full circle.
We can use the following facts to work out angles we don't know:
1. Angles around a single point add up to 3600.
2. Angles on a straight line add up to 1800. So the missing angle below must be 400.
3. Vertically opposite angles are equal. (This is when two straightlines cross!).
4. Angles in a triangle add up to 1800. (See 'Polygons').
5. Angles in a quadrilateral add up to 3600. (See 'Polygons').
When a straight line crosses two parallel lines there are more angle factswe can look for and use!
1. Corresponding angles are equal - these are angles in a letter 'F'.
2. Alternate angles are equal - these are angles in a letter 'Z'.
3. Supplementary angles add up to 1800 - these are angles in a letter 'U' or 'C' (when the 'U' and the 'C' are made of three straight sides, of course).
The diagram below might help you to see this more clearly. Click on thepairs of angles you want to see and they will be shown on the diagram:
When you've learnt all the above facts, the difficult thing is spotting them in the first place! You need to try out your knowledge by practising questions and ignoring any lines that are 'in the way'! Geometry problems will now be a doddle!