Proof Sorter - Sum of an AP

Why do this problem?
This is a self-checking and enjoyable way to learn and to understand the proof of the formula

Possible approach
Not everyone has their maths lessons in a computer room! This exercise can be set for homework if the learners have access to computers at school to do their homework, or at home.

If you want to use this actvity without a computer how about clicking on 'Printable Page' and printing this for yourself. You can then cut out the pieces and rearrange the bits of paper. Of course teachers can photo-copy this for a class.

Key question
Can you explain all the steps in the proof?