Description/Aim

The aim of this activity is to discover that similar right-angled triangles have equal ratios for opposite and hypotenuse, adjacent and hypotenuse and opposite and adjacent. The students use a dynamic geometry package (Cabri geometry) to generate right angled triangles and measure their lengths. Students discover that the sine, cosine and tangent buttons on their calculators equate to these ratios.

Why we like this activity ….

The use of Cabri enables this introduction to trigonometry to move on quickly. I’m sure most of us have used this kind of introduction using paper and pencil before (and been frustrated by the lack of pace and the inaccurate constructions!) Students quite naturally want to play around with the triangle and usually try to produce a triangle with the biggest angle possible which leads into a nice discussion about whether an angle A can be 90°.

How this activity be used ….

The students can work individually but sharing of results is essential. A recap of similar triangles may be useful beforehand. To change the angle they need to double click on the number and toggle up and down.

What to expect when using this activity – from our experience

This activity can be delivered in a 50 minute to 1 hour lesson depending on the ability of the students. Students need not have any prior experience of this software package.

Extra Notes