Basic 1 · Mathematics · Number Operations
Adding and Subtracting Are Related
In this topic you will see how adding and subtracting are linked. You will use this to find missing numbers within 20.
By the end of these notes you should be able to:
- Show that subtraction can be written as addition
- Find a missing number in a number sentence
- Write a family of addition and subtraction facts
What you already know
- Word problems with adding and subtracting
Copy everything that has a blue line on its left into your notebook. The other parts are for reading.
- Strand
- Number
- Sub-strand
- Number Operations
- Indicator
- B1.1.2.2.2 – Use relationship between addition and subtraction to demonstrate understanding of equality for numbers within 20
Introduction
8 + 2 = 10. If you take the 2 back, you have 8 again. Adding and taking away undo each other.
1. Subtraction as Addition
10 – 8 = ? means "8 + what makes 10?"
Example
10 – 8 = ?
- Think: 8 + ? = 10
- Count on from 8 to 10: 9, 10. That is 2.
Answer: 10 – 8 = 2
Example
20 – 12 = ?
- Think: 12 + ? = 20
- Count on from 12 to 20. That is 8.
Answer: 20 – 12 = 8
2. Fact Families
Three numbers can make four facts.
Fact family for 3, 5 and 8
- 3 + 5 = 8
- 5 + 3 = 8
- 8 – 3 = 5
- 8 – 5 = 3
Remember: The biggest number in a fact family is the total.
New words
- related
- Joined or linked together.
- fact family
- Three numbers that make two adding and two taking away facts.
- missing number
- A number we must find.
- total
- The biggest number in a fact family.
Summary
- Subtraction can be written as addition: 10 – 8 is 8 + ? = 10.
- Three numbers make four facts.
- Use adding to check subtraction.
Class activity: Fact Family Houses
- Your teacher gives your group three number cards.
- Make the four facts with bottle tops.
- Write the four facts in your book.
You need: Bottle tops, Bundles of sticks, Exercise books
Exercises
Answer in your exercise book first, then check your answers.
1. 7 + ? = 10
- A. 2
- B. 3
- C. 4
- D. 17
Show answer
Answer: 37 + 3 = 10.2. Which fact belongs to the family 4, 6, 10?
- A. 10 – 4 = 6
- B. 10 + 4 = 14
- C. 6 – 4 = 2
- D. 4 + 4 = 8
Show answer
Answer: 10 – 4 = 6It uses only 4, 6 and 10.3. 9 – 5 = 4 because 5 + 4 = 9.
- A. True
- B. False
Show answer
Answer: TrueAdding checks the subtraction.4. 15 – 5 = 5
- A. True
- B. False
Show answer
Answer: False15 – 5 = 10, because 5 + 10 = 15.5. 12 – ______ = 7
Show answer
Answer: 57 + 5 = 12.6. Write the four facts for 2, 9 and 11.
Show answer
Answer: 2 + 9 = 11, 9 + 2 = 11, 11 – 2 = 9, 11 – 9 = 211 is the total.
Homework
- Write the fact family for 6, 7 and 13.