This is the first lesson to kick off subtraction in my class. I want to make a strong impression on my kids that this is something different that we are beginning and in a small way create a dividing point between the unit on addition we did just before this unit and this unit. Addition is strongly related to subtraction, but I know that first graders usually struggle with subtraction because it is more abstract in thought than addition. Within this unit, in a future lesson, students will need to relate that an addition fact can help them solve subtraction problems, which helps support the deep, algebraic thinking required in Common Core standard 1.OA.B.4. I plan to emphasize that we are going to be detectives and solve problems to find missing pieces. Each part of the problem that we do know is a clue and we have to find the missing part to achieve mastery of the first grade Common Core standard 1.OA.D.8.
First, I will read a Nate the Great book to them. Check the resource section for a picture of our reading time. You can pick any book in the series, I chose Nate the Great and the Big Sniff. I picked Nate the Great because the kids can relate to him. He is a young boy who likes to solve problems. He examines clues and perseveres until the problem is solved. This is exactly what we want our kiddos to do and develop strong mathematical practices.
After I read the book I will show them the pattern to making a detective hat, see the resource section for a printable. I will tell them we need detective hats for what we are getting ready to do in math, solve subtraction problems. Subtraction problems always have a number go away and we want to solve what disappears, 1.OA.1. This is the structure of a subtraction problem and we want them notice it.
Go to the resource section and watch the video showing how excited my students are to begin this lesson. Creating the hats really set a positive tone for the lesson and built in anticipation. It can't get any better than to see them looking forward to what we are getting ready to do in math.
I will be using cubes as a tool for my students to have a concrete method to identify the missing parts of problems. Using objects is a great strategy supported by the Common Core for first graders to solve subtraction and it will assist them in future problems when they must internalize this concrete learning and begin thinking abstractly.
I will get 6 unifix cubes and a small bowl. I will gather my students around a table. I will have a piece of paper close by to record my answers. I will show my students I have 6 cubes to start with. I will have them turn their backs and make sure no one is watching and I will place some of my cubes under the bowl. I will start with putting 2 cubes under the bowl. I will have them turn around and look to see what I have done. I will show them on my paper the following expression:
I want them to help me figure out what is missing. I will show them to start from 2 and count up to find what is missing. Then, I will ask them what can I add to 4 to get 6 as my answer.
We will go on to placing 1 under, 3 under, and 0 under. For each of these new problems, I will always follow with what addition equation do we know that can help us solve the problem. I want teach them explicitly that not only can I view the problem as an opportunity to count up and solve, but there is an addition fact that is related to this subtraction equation.
Then I will add another cube and show them 7 cubes altogether. We will repeat the same process and identify missing parts of 7.
Each time we solve a missing part, I will write down the expression for the problem and show my students. It is a precise process and I want them to notice that accuracy in recording my knowledge really counts or I will not create the correct problem.
I will not identify every single missing part expression because of time restraints and I want to leave it open ended to see if they identify ones I did not.
When they finish their independent practice I will use their list of problems to find the ones I did not find with them and show the class what great detectives they were today.
I will give the students a partner and each set of partners will get 6 unifix cubes.
First, one student closes their eyes. Then, the other student places some of the cubes under the bowl. Next, they work through the problem and solve to find out how many are under the bowl.
Finally, they write down their answer. I will ask each student to write their own problems and the partners are not finished until each one has 4 problems written. I will ask them to come get an extra cube to begin playing for missing parts of 7.
Check the resource section for videos of students solving problems for missing parts of 6 and 7.
After the game, I will pass out the Practicing 6 and 7 worksheet to each student. These are just straight forward subtraction problems for them to begin practicing using their fingers to solve take away. They are written in vertical format because they learned through addition lessons and today's subtraction lesson that the answer goes after the equals sign. The most common problem, see resource section, for first graders when transitioning from addition to math, is they want to continue solving their problems by adding. You will want to check their papers as soon as possible for this and point out the minus sign. They must pay attention to the structure of the problem to build the correct model with their fingers or any other manipulatives you offer them.
To close out the lesson, I ask 3 to 5 students to share out about a subtraction problem they solved and to demonstrate what strategy they used to solve it.