Articulating, Editing, and Refining Conjectures in a Second or Third Grade Classroom

Processes

Content

Teacher planning and reflection Assessment Classroom view
Mathematical Reasoning and proof Proving an argument involves a conjecture about what is believe to be true for all cases, articulating that idea, refining it, and editing it until it is acceptable or proving it is false.
Communication - It's important to use precise language in stating mathematical ideas so people know exactly what is meant and will be able to understand and communicate with one another.

Zero

Properties of zero

Property of zero for addition

 

Teacher has the following outcome - students will construct a mathematical proof by creating ideas, articulating what they believe to be possible conjectures, refine them through reasoning and comparison to information, and edit conjectures to make them more accurate and meaningful as new ideas are presented.  

Activity - Discussion about the accuracy mathematical statements written in a TRUE and FALSE format.

The following scenario is from Thinking Mathematically: Integrating arithmetic & Algebra in Elmentary School. by Carpenter, Franke, & Levi.

 

 

 

Decides to assess or review what the students know about zero and adding zero to another number.
He doesn't press the children to make a generalization.
He gives them another number sentence for the same principle, but in this case the number sentence is false.

Diagnositic: Content
Properties of zero
Property of zero for addition


Mr. C. - Writes [58 + 0 = 58] on the board
How about this number sentence? Is it true or false?
Children - True. True.

   

Pressing further to see how strong or fragile the student's understanding of zero and the property of zero for addition.

Uses tranformation one of the mental operations that concrete operational thinkers are begining to develop an understanding that can be used to transform things in different ways. This way is from the infintisimle small to the infinite large.

Diagnositic, formative, summative, and generative: Content
Properties of zero
Property of zero for addition

If all the students are understanding as well as those speaking, then it probably is generative or summative if large numbers and zero were discussed before. If not, then other students might still be in the formative and some may be having the "light come on" and it could be summative.

Mr. C. - How about this one? [Writes 78 + 49 = 78]
Children - False! No, no false! No way!

Mr. C. - Why is it false?
Jenny - Because its the same number as in the beginning, and you already put more with it, so it would have to be higher than the number you started with.
Mike - Unless it was 78 plus zero equals 78. That would be right.

Mr. C. - Why is that true? We added something.
Sieve - But that something is like nothing. Zero is nothing.
Mr. C. - Is that always going to work'?
Lynn - If you want to start with a number and end with a number, and you do a number sentence, you should always put a zero. Since you wrote 78 plus 49 equals 78, you have to change a 49 to zero to equal 78, because if you want the same answer as the first number and the last number, you have to make a zero in between.
Mr. C. - So do you think that will always work with zero? How about this one? [Writes 789,564 + 0 = 789,564]
Children - That's true.

   

Satisfied with their understanding he presses on with what he has planned for reasoning and proof by diagnosing what students know.

Of to a good start the discussion gets side tracked awhile with the idea of zero as part of a number rather than a value. Mr. C aware of how fragile students understanding is returns to helping to facilitate learning about numbers (content of mathematics).

 

Diagnositic: Proces
Proof
- Proving an argument involves a conjecture about what is believe to be true for all cases, articulating that idea, refining it, and editing it until it is acceptable or proving it is false.

Mr.C - How do you know that is true? Have you ever done that?
Ann - I will tell you. All those numbers plus zero, you won't add anything, so it would be the same number.

Mr. C. - So we kind of have a rule here. Don’t we? What’s the rule?

Ann - Anything with a zero can be the right answer.
Mike - No, because it was 100 plus 100 and that’s 200.
Jenny - That's not what we are talking about. It doesn’t have just plain zero.
Ann - I said!, umm, if you have a zero in it, it can't be like 100, because you want just plain zero, like 0 plus 7 equals 7.

After some additional discussion to clarify what the children are talking about the number zero not zero in numbers like 20 or 500, the children are challenged to state a rule that they could share with the rest of the class.

  Addition of zero and zeroes in a numeral are different. (this can be problematic for students at this age, since place value is probably still a year away) Satisfied, again, with their understanding he presses on with what he has planned for reasoning and proof by refturning to formative assessment mixed with ideas being suggested by what students know or are discovering.

Diagnositic, formative, summative, and generative: Content
Properties of zero
Property of zero for addition

Going through a quick cycle of formative assessment, summative, and genertative before returning to processes and proof, when satisfied that students understand.

Ellen - When you put zero with one other number, just one zero with the other number, it equals the other number.
Steve - Not true.

Mr.C. - Wait let me make sure I got it. You said, "If' you have a plane zero with another number." With another number like - like just sitting next to the number?
Ellen - No, added with another number, it equals that number.
Mr.C. - Okay, So what do we want to say here? Ann?

   

Uses vocabulary "conjecture" but does so nonchalantly. This is a sort of priming of the pump for another day.

 

 

Reviews today's / yesterdays activities and is probably pretty pleased. Decides that the conjectures should be shared with the rest of the class, and the sheets put up on the wall.

Formative and moves to summative: Process
Proof

Ann - Zero added with another number equals that other number.
Mr.C. - Is that always going to be true for any number, even really big numbers that we haven’t tried?

Children - Yes! Yes!
Mr. C. - How do you know that? Ellen.

Ellen - Because zero is nothing. You're not putting anything with the number, so it is still the same.
Mr. C. - Do you all agree?
Children - Yes

Mr. C. - So we have a conjecture here about adding zero to any number. Can anybody think of another different conjecture about zero that is true for all numbers?
Lynn - I can. You can take away zero from a number, and you still get that number back.

Mr. C. - Is that always going to be true for every number?
Children - Yes.

Mr. C. - How do you know? Steve.
Steve - Because, it is just like the last one only it's take away. You are not taking anything away.
Mr. C. - We should write these conjectures down so we can remember them and so we can share them with the rest of the class.

Mr. C. - Writes each conjecture on a separate sheet of paper.

Zero added with another number equals that number.
You can take away zero from a number and you get that number back.

Day 2
  Property of zero for subtraction

Reviews today's / yesterdays activities and is probably pretty pleased. Decides that the conjectures should be shared with the rest of the class, and the sheets put up on the wall.


Decides to start with an additional true/false number sentences and if students demonstrate a conceptual understanding of conjecture he will introduce the term conjecture by using it in context.

Diagnostic or review, move to formative for conjecture and proof

Generative for Property of zero for subtraction

Summative and or Generative for conjecture

Mr. C. - How about this one, is this true or fake? [Writes 785 – 785 = 0]
Class - True

Mr. C. - Those are pretty big numbers. Are you sure?
Sally -  Yeah Whenever you lake a number away from itself, you get zero.
Ann - [Excitedly] Hey, we have another conjecture here. A number minus the same number is zero.
Mr. C. - Are you sure that's always true? How do you know? If you have some things and you take them all away, there's none left. It's zero.

Mr. C. - Write his conjecture (A number minus the same number is zero.) on another sheet of paper and adds it to the two conjectures already posted on the wall.

Dr. Robert Sweetland's Notes ©