My Yr11 class have had a really excellent week understanding algebraic proof through Triangle Numbers. Many thanks to the excellent Don Steward whose ideas formed the basis for the learning. I am particularly pleased with the collaborative yet focussed atmosphere in the class when the students are hooked on the maths:

We started by sorting various statements (pictures, words, calculations and algebra). The four things below all say the same thing, in different ways. Lesson here.

At the end of each lesson students would hand to me a write-up of a proof they had thought about in the lesson. I marked the proofs on the following criteria, and made the marks public to the class (projected onto whiteboard) so that students knew who to turn to for support.