Lesson: Bar Model and Addition vs. Subtraction, Day 6
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Lesson Objective
Mathematicians decide whether to use an addition or subtraction bar model to represent a number sentence.
Lesson Plan
Edward W. Brooke Charter School
Unit One: Add and Subtract within 100
Lesson: 2.6
Teaching Point: Mathematicians decide whether to use an addition or subtraction bar model to represent a number sentence.
Materials:
 2.6 Problem Solving Task
 2.6 Independent Practice
 Problem solving kits
Problem Solving Task (5 minutes)
 See Problem Solving Task 2.6
Teaching (20 minutes)
 Have students share how they solved the problem solving task with a partner. *You might want to direct the turn and talk by giving separate, specific prompts related to the steps students had to follow to solve the task:
 Turn and read the number sentence you had to solve to your partner.
 Turn and tell your partner what the two parts are in the number sentence that you’re going to put together/what the total in the number sentence is and what the part is that you are going to take away.
 Turn and tell your partner how many dots you drew for each part and where you put a question mark/how many dots you drew in the bar for the total, how many you dots you separated from the total and where you put your question mark.
 Turn and tell your partner how many dots you had when you counted the 2 parts together/how many dots were left when you took a part away from the total.
 Turn and tell your partner the other three number sentences in the fact family.
 Have a whole class discussion about the problem solving task.
Discussion:
For the addition number sentence:
 Highlight that this is an addition number sentence, so we are going to fill in the bar model with the information we have in an addition number sentence (two parts that need to be put together). Call on students to share what they drew and labeled for the first part (5 circles/5) and the second part (2 circles/2)
 Ask students where they put the question mark and why (we labeled the total with a question mark because this is an addition number sentence. We have 2 parts and need to put them together to get a total. Since the total is what we don’t know and are going to solve to figure out, we label the total with a question mark).
 Go over the remaining three number sentences in the fat family, highlighting how the bar model illustrates why these number sentences are true and make sense.
For the subtraction number sentence:
 Highlight that this is a subtraction number sentence, so we are going to fill in and label this bar model differently because we start with different information in a subtraction number sentence (a total and part that we are taking away).
 Call on students to share what they drew and labeled for the total (4 circles/4).
 Call on students to share how many circles they separated with line and labeled for the part being taken away (3 circles/3).
 Ask students where they put the question mark and why (we labeled the part that’s left with a question mark because this is a subtraction number sentence. We have the total and one of the parts that we are going to take away from the total to get the other part that’s left. Since the other part that’s left is what we don’t know and are to solve to figure out, we label the total with a question mark).
 Explain, “These two bar models are different because in addition and subtraction we are starting with different information (either two parts or a total and a part), doing different things with that information (either putting two parts together or taking part away from a total), and solving to find different information (either a total or a part that’s left). When mathematicians are drawing bar models to help them represent and understand addition or subtraction number sentences, they have to be very careful that they are filling their bar models with the information they have in the correct order and correct places. If we are solving an addition number sentence, does it make sense to try to start by filling in the bar model with the total? No! We don’t even have the total yet – that’s what we’re trying to find out! If we are solving a subtraction number sentence, does it make sense to try to start by filling in the bar model with two parts? No! We don’t even have the other part yet – that’s what we’re trying to find out!
 Have students get their slates and markers. “Now you are all going to try this out!” Write addition or subtraction number sentences on the board. Ask students question about the information we are starting with, where that information needs to go in the bar model, and what information we’re trying to find out. Have students draw bar models to represent the different types of problems.
Independent Practice:
 See 2.6 Independent Practice. Since students will be drawing their own bar models instead of filling them in, you might want to do the first one on the rug together.
Lesson Resources
2.6 Independent Practice.doc 
773

2.6 Problem Solving Task.doc 
359

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