Teaching note:
This rule is usually known as 'angles on a straight line sum to 180 degrees' but you need to take care that students don't over-generalise. It is a common misconception for learners to add all the angles on any given line, even when they occur at different points. Another Geogebra page is supplied entitled 'angles at points on a line' for you to dispel this. For this reason in my classroom we call this 'angles [i]at a point [/i]on a straight line sum to 180 degrees' to emphasise that the angles must be coincident.
Key questions:
[*]Do angles on a straight line [i]always[/i] add up to 180?
[*]Why or why not?
[*]Do the angles [b]at a point [/b]on a straight line [i]always[/i] add up to 180?
[*]Why or why not?
