Angles in a triangle

A complete lesson on the interior angle sum of a triangle.

Activities included:

Starter:
Some simple recap questions on angles on a line, as this rule will used to ‘show’ why the interior angle sum for a triangle is 180.

Main:

• A nice animation showing a smiley moving around the perimeter of a triangle, turning through the interior angles until it gets back to where it started. It completes a half turn and so demonstrates the rule. This is followed up by instructions for the more common method of pupils drawing a triangle, marking the corners, cutting them out and arranging them to form a straight line. This is also animated nicely.
• A few basic questions for pupils to try, a quick reminder of the meaning of scalene, isosceles and equilateral (I would do a lesson on triangle types before doing interior angle sum), then pupils do more basic calculations (two angles are directly given), but also have to identify what type of triangles they get.
• An extended set of examples and non-examples with trickier isosceles triangle questions, followed by two sets of questions. The first are standard questions with one angle and side facts given, the second where pupils discuss whether triangles are possible, based on the information given.
• A possible extension task is also described, that has a lot of scope for further exploration.

Plenary
A link to an online geogebra file (no software needed, just click on the hyperlink).
This shows a triangle whose points can be moved dynamically, whilst showing the exact size of each angle and a nice graphic of the angles forming a straight line. I’ve listed some probing questions that could be used at this point, depending on the class.

I’ve included key questions and ideas in the notes box.

Optional, printable worksheets and answers included.

Please do review if you buy as any feedback is helpful and appreciated!