Making the Minus Make Sense

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Objective

SWBAT explain what each mathematical symbol (+ and -) mean. SWBAT represent what is happening in a story problem with a subtraction equation.

Big Idea

Students take on the challenge of reasoning abstractly (MP2) and using the minus symbol to represent their story problems in equations!

Setting Up the Learning

5 minutes

In first grade, students are expected to be able to solve a variety of types of addition and subtraction story problems and represent them using an equation (1.OA.1). This lesson focuses on the story problem that some students tend to struggle with a little bit more-subtraction!

Review:

Yesterday we discussed how mathematicians use the + and - to represent their thinking. Today we are going to zoom in on how to write equations that have a minus symbol.

Connect:

Using symbols helps us write all of our thinking really quickly. It helps us write using only numbers and symbols-no words!

Objective: How do I write a matching subtraction equation?

Opening Discussion

10 minutes

I'll start this lesson by sharing out one of the subtraction problems from yesterday's lesson. (Go to this link for how I introduced the + and - symbol together).

I'll choose a direct modeling strategy-one where a student acted out the problem with cubes to share. The focus of the share is on what we did first, next and last in the problem to provide a scaffold for how we write the equation.

Read problem: You have 15 soccer balls in your shopping cart. You give 4 soccer balls away. How many soccer balls are in your shopping cart now?

Guiding Questions before I share the strategy: Visualizing problems is KEY to helping kids understand why they use the + or - symbol, and what each number means. These questions help students "make sense of the problem" (MP1) before they attempt to solve, represent their strategy and write an equation to match.

• Reread the problem to your partner.
• Close your eyes and visualize what is happening. Will you have more or less soccer balls in your shopping cart in the end?
• Are we putting together or taking away? What symbol do we use to represent that?

After I share the strategy, we will focus on how to write the equation.

Scaffolding Questions: These questions help set up students to think about retelling the problem, which actually will help guide them to being able to write the equation to match it!

• What did you do first? What happened next? What happened last?
• Let's write that in numbers and symbols.

Student Work Time and Strategy Share

15 minutes

Present new problem: There are 10 frogs in the pond. 3 of them hop away. How

many frogs are in the pond now?

Guiding focus before students go and solve: Students retell the story to a partner and determine if it is “put together” or “take away”.

Student Work Time:

Students work for 5-6 min on solving and representing how they solved the problem in their math journals.

Strategy Share:

Students first share their own strategies from their math journals. See attached video for an example of a partner share! Having students discuss their thinking pushes them to CCSS.Math.Practice.MP3 Construct viable arguments and critique the reasoning of others.

After students partner share, I'll choose one strategy to share. Because students have been working on subtraction strategies for a couple of days, I'll quickly share out a more difficult strategy than just cubes, such as counting backwards or using a number line.

Guiding Questions:

• Did we put groups together or take away?
• What symbol do we need to use then?
• What number will I write first? What symbol? How many did I take away?

Independent Practice

15 minutes

See this video for how I differentiated the independent practice!

See attached documents! I left the numbers out of the story problems so teachers can write in numbers that work best for theirs students.

Closing

5 minutes

Students will end the lesson by doing a quick partner strategy share, focusing on how they wrote a matching equation.