Strategies for Subtraction Facts

Est. Class Sessions: 2–3

Developing the Lesson

Part 2: Introduce the Triangle Flash Cards for Subtraction

Demonstrate Using Triangle Flash Cards for Subtraction. Display the Large Triangle Flash Card Master to introduce how to use the triangle flash cards for subtraction. Write 4 in the circle, 2 in the square, and 6 in the shaded corner. See Figure 5. Ask a student to review how the cards are used to practice addition facts. Have another student record the appropriate number sentences on the board (4 + 2 = 6,
2 + 4 = 6). Repeat this a few times with other addition facts.

Next, ask how the same card might be used to practice subtraction facts. Some students may suggest beginning with the sum and covering one of the other corners, resulting in a subtraction problem. Talk with students about how the number sentence might be recorded. For example, if there is a 3 in the circle, a 1 in the square, and a 4 in the third corner, the subtraction sentences can be 4 − 3 = 1 (covering the 1) or
4 − 1 = 3 (covering the 3). If students do not suggest a way to use the cards, demonstrate this for them.

Ask:

Is it possible to use one card for more than one math fact? (yes)
How many? (four)
What are the number sentences? (3 + 1 = 4;
1 + 3 = 4; 4 − 3 = 1; 4 − 1 = 3)

Remind students that the subtraction facts are related to the addition facts. We call the four related facts a fact family. Ask a student to tell you a number sentence using the numbers 4, 1, and 5. Write the sentence on a display.

Ask:

What is a different number sentence using 4, 1, and 5?

Continue until all four number sentences are written for the students to see.

After each sentence is given, ask:

How is this new sentence different from the ones we have already? (Possible response: 5 − 1 = 4 is a subtraction sentence, and 4 + 1 = 5 is an addition sentence.)
How are these number sentences the same? (They are both from the same fact family. They both have the same numbers.)
When you write a subtraction number sentence which number should you write first? (the largest number)

Choose three different numbers and repeat the conversation.

Then choose numbers such as 2, 2, and 4.

Ask:

How many number sentences can we write for these numbers? (2)
Why are there only 2 number sentences, an addition number sentence and a subtraction number sentence? (This is a double, so 2 + 2 = 4 will not look different if we use the turn-around fact.)

Sort Triangle Flash Cards. Distribute a copy of the Triangle Flash Cards: Group A to every student and ask them to cut out the cards. After the cards are cut out, have students work in pairs to practice the subtraction facts. Explain the steps for students to sort their flash cards:

One partner chooses a flash card and covers the corner with a number in the circle. This number is the answer to a subtraction fact.
The other partner uses the two visible numbers to solve a subtraction fact and says the resulting subtraction sentence.
As the students work through the cards, they use the Sorting Board to divide the cards into three piles: “Facts I Know Quickly,” “Facts I Can Figure Out,” and “Facts I Need to Learn.”
If a student gives the correct answer quickly, the card is placed in the “Facts I Know Quickly” pile. If the student answers correctly after thinking through a strategy, it is placed in the “Facts I Can Figure Out” pile. If the student gives the incorrect answer, it is placed in the “Facts I Need to Learn” pile.
The student displaying the flash cards helps his or her partner find the facts from the “Facts I Know Quickly“ pile on his or her Subtraction Facts I Know chart so he or she can circle the known facts easily.
Students place the cards in one pile and go through the same steps again, this time covering the numbers in the squares. After the facts that the student knows quickly are circled on the Subtraction Facts I Know chart, the cards can be clipped together and placed in an envelope for storage.
Partners switch roles and repeat the procedure.

Complete the lesson by discussing students’ strategies for learning the facts. Include how knowing an addition fact helps in learning a subtraction fact.

Use the Fact Families with Subtraction Triangles pages in the Student Activity Book to assess students’ fluency with the subtraction facts for Group A.

Students use the Triangle Flash Cards: Group A, the Sorting Board, and the Subtraction Facts I Know chart to assess their fluency with the subtraction facts related to the addition facts in Group A (1 − 0, 1 − 1, 2 − 0, 2 − 1, 2 − 2, 3 − 1, 3 − 2,
4 − 1, 4 − 2, 4 − 3, 5 − 2, 5 − 3, 6 − 2, 6 − 4) [E6].

Ask students to store their Sorting Board and Subtraction Facts I Know chart from the Student Activity Book in a safe and central place in the classroom. As students progress through each unit, they will be asked to assess and track their progress. Copy the Triangle Flash Cards: Note Home Homework Masters to provide a set of directions and tools for use at home.

Use the Fact Families with Subtraction Triangles pages in the Student Activity Book to assess students’ abilities to demonstrate fluency with the subtraction facts related to the addition facts in Group A (1 − 0, 1 − 1, 2 − 0, 2 − 1, 2 − 2,
3 − 1, 3 − 2, 4 − 1, 4 − 2, 4 − 3, 5 − 2, 5 − 3, 6 − 2, 6 − 4) [E6] and determine the unknown number in an addition or subtraction sentence relating three whole numbers for the facts in Group A [E7].

ADVENTURE BOOK: