What the lesson is about
This is the first lesson in a unit introducing pupils to the basics of trigonometry and can be used for key stage 3 and 4 pupils. Pupils practise enlarging triangles using different scale factors to produce similar shapes. The ratio between the corresponding sides of the shapes is then emphasised to inform further work.
Aims: Pupils will ...
- understand how to enlarge a shape using a centre of enlargement and a scale factor;
- know that enlargements produce similar shapes;
- know that similar shapes have corresponding sides in the same ratio;
- be able to work out ratios of corresponding sides.
Getting started
Print the enlargement sheets (available at www.tes.co.uk) and cut them up. Divide the class into small groups or pairs and ask them to match the shapes that they believe show enlargements. How many can they find? How do they know that one shape is an enlargement of another? Let pupils discuss their ideas but do not give them the answers and leave the shapes on their desks to look at later in the lesson.
Show the PowerPoint presentation on shapes (also available at the TES website) to review how to enlarge a shape using a centre and a scale factor.
Hand out the enlargement worksheet (also from the site), which shows five triangles with a centre of enlargement. Ask the pupils to enlarge the shapes by the factor shown (two, three, four, a half).
Return to the PowerPoint and focus on slides six and seven, showing that the ratios of corresponding sides of similar shapes are equal.
Ask pupils to test this for themselves on the worksheets to see how accurate they have been with their enlarging and to make sure they understand what corresponding sides are and that they will be equal.
Taking it further
Return to the matching shapes activity and ask the pupils to reassess their answers. Get them to use the ratios of corresponding sides to check those they have matched and see if they can find any others that match.
Where to find it
The complete five-lesson unit, plus supporting resources including printable sheets and PowerPoints, was uploaded by Lez and can be found at www.tes.co.ukenlargement-similar-triangles.
