Even and Odd Numbers, Part 2: A Journey into the Algebraic Thinking Practice of Justification
By Isil Isler, Ana Stephens, and Hannah Kang, posted
February 1, 2016 –
In the previous blog post we provided some ideas about
algebraic thinking and mathematical justification. We also introduced the
Brian knows that anytime you add three odd numbers, you will
always get an odd number. Explain why this is always true.
In what follows, we share a sample of student responses to
the sum of three odd numbers task and use Carpenter and his colleagues’ (2003)
justification framework to identify the nature of their responses.
Before reading further, please think about the nature of
these responses with regard to the justification framework. What do these
responses tell us in terms of how students reason about the Sum of Three Odd
Numbers task? Do these arguments show that the conjecture is always true? Which
ones would you hope to see your students provide to show that the conjecture is
Jordan’s work is an example of justification by example.
This is, by far, the most common type of justification we observed in our
study. After trying out a few examples, students usually conclude that a given
conjecture—in this case that the sum of three odd numbers is an odd number—is
true for all numbers.
Next, let’s focus on Alexis’s response. This response is different
from Jordan’s argument. It uses specific numbers but makes use of the structure
of the numbers rather than their values. Alexis represented specific numbers
using circles, split them into pairs, and produced a general argument based on
the definition of even and odd numbers and the structure of the representation.
The specific numbers are thus used in a “generic” way. This response can
therefore be categorized as a generalizable argument in Carpenter and her
colleagues’ (2003) framework. This type of general argument is called representation-based proof and is
accessible to elementary school students (Russell, Schifter, and Bastable 2011;
Schifter 2009). During our intervention and on our assessments, we found that
our students benefited from using representations because they helped students focus
on underlying structure and reflect on why.
Finally, let’s focus on Peggy’s response. How is it
different from Jordan’s and Alexis’s responses? Peggy’s argument is not
dependent on specific numbers; rather, it is built on generalizations about
sums of even and odd numbers. Therefore, this argument is also categorized as a
general argument. The argument uses a chain of accepted arguments to prove that
the conjecture is true. In other words, the student used statements that were
already justified in the classroom (i.e., the sum of two odd numbers is an even
number, and the sum of an even number and an odd number is an odd number) to
justify another conjecture—that the sum of three odd numbers is odd. We encourage
you to support your students to develop conjectures based on previously
justified conjectures. As was the case with representation-based proofs, we
found that elementary school students in our early algebra intervention were
able to produce general arguments using justified conjectures.
We can help our students move beyond examples-based
reasoning by asking them why they think a given conjecture is always true and
encouraging them to look for and make use
of structure (SMP 7).
Here are some suggestions to help you engage your students in justifying and
We encourage you to try the Sums of Evens and Odds activity
and/or the Sum of Three Odd Numbers task with your students and share with us what
you learn! Post your comments below or share your thoughts on Twitter
@TCM_at_NCTM using #TCMtalk.
P., Megan Loef Franke, and Linda Levi. 2003. Thinking Mathematically: Integrating Arithmetic and Algebra in the
Elementary School. Portsmouth, NH: Heinemann.
State Standards Initiative (CCSSI). 2010. Common Core State Standards for
Mathematics (CCSSM). Washington, DC: National Governors Association Center for
Best Practices and the Council of Chief State School Officers. http://www.corestandards.org/wp-content/uploads/Math_Standards.pdf
Kaput, James J.
2008. “What Is Algebra? What Is Algebraic Reasoning?” In Algebra in the Early Grades, edited by David W. Carraher and Maria
L. Blanton, pp. 5–17. New York: Lawrence Erlbaum Associates.
Knuth, Eric J.,
Jeffrey M. Choppin, and Kristen N. Bieda. “Middle School Students’ Production
of Mathematical Justifications.” 2009. In Teaching
and Learning Proof across the Grades: A K–16 Perspective, edited by Despina
A. Stylianou, Maria L. Blanton, and Eric J. Knuth, pp. 153-170. Studies in Mathematical
Thinking and Learning Series. New York: Routledge.
Council of Teachers of Mathematics (NCTM). 2011. Developing Essential Understanding of Algebraic Thinking for Teaching
Mathematics in Grades 3–5, edited by Maria Blanton, Linda
Levi, Terry Crites, and Barbara Dougherty. Essential Understanding Series. Reston,
Jo, Deborah Schifter, and Virginia Bastable. 2011. Connecting Arithmetic to Algebra: Strategies for Building Algebraic
Thinking in the Elementary Grades. Portsmouth, NH: Heinemann.
2009. “Representation-Based Proof in the Elementary Grades.” In Teaching and Learning Proof across the
Grades: A K-16 Perspective, edited by Despina A. Stylianou, Maria L.
Blanton, and Eric J. Knuth, pp. 87–101. New York: Routledge.
Dr. Isil Isler, firstname.lastname@example.org,
is a recent graduate of the University of Wisconsin–Madison. She is interested
in algebraic thinking, and reasoning and proof in the elementary and middle
grades. Dr. Ana Stephens, email@example.com,
is an associate researcher at the Wisconsin Center for Education Research at
the University of Wisconsin–Madison. She is interested in the development of
students’ and teachers’ algebraic reasoning and helping teachers focus on
students’ mathematical thinking. Hannah Kang, firstname.lastname@example.org,
is a Master’s of Science candidate at the University of Wisconsin-Madison. She
is interested in equity and diversity issues within mathematics education, as
well as students’ algebraic thinking.
The research reported here was supported in part by the
National Science Foundation (NSF) under DRK-12 Award No. 1219605/06. Any
opinions, findings, and conclusions or recommendations expressed in this blog
are those of the authors and do not necessarily reflect the views of NSF.