3. Angles in a triangle Look at triangle ABC shown below: c
Photo credit: © Freddy Eliasson 2010, Shutterstock.com
For any triangle, the interior angles will add up to 180
How do we know this?
How can we prove that this is true?

4. Interior angles in a triangle

5. Exterior angles of a triangle

6. Interior and exterior angles in a triangle
Teacher notes
Drag the vertices of the triangle to show that the exterior angle is equal to the sum of the opposite interior angles.
Hide angles by clicking on them and ask pupils to calculate their sizes.
Be aware that the angles are rounded to the nearest degree and that this may cause a slight error of ±1°.

7. Calculating angles