Think About Subtracting Nothing

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Objective

Students will be able to subtract 0, 1 and 2 from a number and apply this as 0 less than, 1 less than, and 2 less than within 20.

Big Idea

The big idea of this lesson is thinking about subtraction facts with 0, 1, and 2 as amounts less than a number.

Activator and Materials

10 minutes

Activator:

Begin the lesson by having the students review what they know about subtraction.  If necessary, guide the students to discuss how subtraction has a subtrahend (the larger number), a minuend (the smaller number), and the difference (the answer), as well as the idea that subtraction can be thought of as either taking something away and comparing two amounts.

Materials:

A great thing about this lesson is that it doesn't require a lot of materials - just connecting cubes, the practice sheet, and two dice. 

Develop the Concept

20 minutes

As we begin, I tell the students to create a train of 6 connecting cubes.  Then, I ask them to use that train of connecting cubes to show zero less. 

What would the number sentence be for this problem? I take a few students responses, and continue this activity (within 10) with several other examples done as a class. Using the connecting cubes this way allows me to see immediately who is "getting it". 

Practice the Concept

20 minutes

For the group activity, we'll subtract 0, 1, and 2 only. Each pair of students need two dice.  Although students share the dice, they should be doing a great deal of the work independently.

They would first roll both dice and find the sum of the two dice.  They would then use that sum to complete the sentences and number sentences  

0 less than 7 is 7                    7 - 0  =  7

The reason that I have students complete both a number sentence and a "sentence" is so that they understand the mathematical language of subtraction and recognize how the number sentence is set up differently than the regular sentence.

Summarizer

10 minutes

As students complete their work, I have them come back together as a class to share about what they learned about subtracting 0, 1, and 2.  I ask the students how this could help them in their math fact fluency.  The CCSS aim is to develop fluency in the recall of basic math facts.  

In order for a student to develop a strategy, they need to be able to understand how it is useful. So I prompt them to make those connections using questions.

How could it help them with word problems?  How would you use this in your real life? 

I find that even though these questions are open-ended, this culminating activity can provide you with meaningful information about possible gaps in learning, and display student thinking but demonstrating how well they will utilize the strategy.