Download The Aim of this lesson is to show you how to use a

Cabri Worksheet
Angles in a triangle
South Hunsley School
Mathematics Department
The Aim of this lesson is to show you how to use a dynamic geometry
software package called Cabri Geometre. A dynamic geometry software
package allows you to do the sort of constructions you can do with a pencil,
ruler and compasses, but because it is ‘dynamic’, you can move parts of the
construction around to see what happens if things change.
Each section will guide you through a construction and then ask you some questions about
it. It is important that you always label any point that you draw. You may need to use the
‘Cabri Help Sheet’.
Construction
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Place three points anywhere on the page.
Label the points A, B and C.
Draw Triangle ABC.
Mark Angle ABC. Mark Angle BCA. Mark Angle CAB.
Measure Angle ABC. Measure Angle BCA. Measure Angle CAB.
 Your construction is now complete. You can move points A, B and C using
the Arrow tool.
Show your construction to your teacher and ask if you can print it. Now answer the
questions. You can write the answers on your printout.
1. What do you notice when you add up the Angles?
2. Is this the same if you move the Points around?
3. Why don’t the Angles always add up to exactly 180°?
Fact Time:
 The angles in a triangle always add up to 180°
 In an equilateral triangle each angle is 60°
Can you make your triangle into an equilateral triangle?