Angles in Triangles
A. Angles in Triangles
In the Warm Up you identified a relationship for the interior angles sum for a triangle.
Use the Triangle Angle Sum applet to check if your relationship works for other triangles.
Move the points A, B and C to see the sum of angles in a triangle for different triangles.
1 February 2013, Created with GeoGebra
You have seen inductive evidence that the three interior angles in any triangle will add up to 180°. This can be proven using concepts from the last lesson.
Provide an explanation for each diagram to complete a proof that the interior angles in any triangle sum to 180°.
| Diagram | Proof Explanation |
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Hint: The symbols are showing two pairs of equal angles. |
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Compare your proof to a partner's proof or to an existing proof by entering "triangle angle sum proof" into your favourite search engine.
You can now conclude with certainty that the sum of the interior angles in any triangle is 180°. This fact can now be used to solve other problems.