 Proposition 31 in book 6 of Euclid's Elements describes the similarity proof, which states that the squares of each side can be replaced by shapes that are similar to each other and the proof still works.The second module created was "The Story of Pi", in 1989, and describes the mathematical constant pi and its history.The first letter in the Greek word for "perimeter" (περίμετρος) is , known in English as "pi".