## Lesson: Inverse Relationship Between Addition and Subtraction Introducing the Concept

Your children have been working with addition and subtraction facts. Today they are going to work with related facts.

Materials: 2 colors of counters, 10 of each; number cards 2 – 12 (or numbers the class is working with); paper and pencil

Preparation: Place the number cards facedown on a table along with the two colors of counters.

Prerequisite Skills and Concepts: Children should understand addition and subtraction. They should be able to add sums to 10 and subtract from 10 or less.

Review with children related facts. Have a volunteer draw a card and hold it up for the class to see. Have another volunteer take counters from both colors to equal the amount on the number card and place them in two groups. Ask the class to count how many counters in each group.

• Ask: What addition fact can we write for what we have just done? What sign will you use?
Elicit from the children that they can add the number of the two groups of counters, and that the result equals the number on the card. They will use the plus sign.
• Write the number sentence on the chalkboard as the children write it on their papers.
• Ask: How can we undo what we just did?
Children should say that the counters can be put together and then one group can be taken away.
• Have a child act this out with counters. Then have the class write the number sentence to show what they did below their addition number sentence.
• Ask: What number sentence did you write? What sign did you use?
Children should say that they wrote the total number minus the number in the group taken away and that the result equals the number of counters left. They used the minus sign.