Parallel geometry tasks with four levels of complexity involve students in writing and understanding proof.
Improving Student Reasoning in Geometry
Bobson Wong and Larisa Bukalov
Informing Practice: Examples as Tools for Constructing Justifications
research matters for teachers
Kristen N. Bieda and Jerilynn Lepak
Research explores how to help students build from, instead of building with, examples when justifying mathematical ideas.
Proofs without Words: A Visual Application of Reasoning and Proof
Carol J. Bell
Reasoning and Proof is one of the process standards set forth in NCTM's principles and standards for school mathematics (2000).