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# Convince me

A proof establishes the validity of a conjecture for **every possible variation** of the situation in which it could arise. Initially, students may consider that a demonstration is sufficient.

Watch the video *Demonstration of the angle sum of a triangle* which provides evidence to conjecture that the angle sum of a triangle is 180°. Students could try this activity for themselves.

In contrast to a demonstration, a geometric proof needs to be an orderly progression of statements which link previously established facts each supported with reasons. Watch the *Two Proofs of the Angle Sum of a Triangle* slide presentation.

**Top Drawer Teachers: Two proofs of the angle sum of a triangle**from

**The Australian Association of Mathematics Teachers (AAMT) Inc.**

You can also download the accompanying *Angle Sum of a Triangle: Student Worksheet*.

A dynamic diagram of the angle sum of a triangle is available on Geogebra Tube.

It is important to explain to students that a proof is incorrect if there is:

- an error in the underlying assumptions
- a flaw in the logic of the reasoning.