Lesson Notes By Weeks and Term v3 - Primary 1
Addition II
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Addition II
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Subject: General Mathematics
Class: Primary 1
Term: 1st Term
Week: 9
Theme: Basic Operations
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Watch on YouTubefind missing numbers in a statement; cross-check the correctness of addition in everyday life.
Re-adding Carefully Explanation: Simply performing the addition again slowly and carefully, perhaps using a different counting strategy (e.g., starting from the larger number).
Example: A student calculates 5 + 3 =
8. To cross-check, they can re-count: "Start with 5, then count 3 more: 6, 7, 8." The answer is still
8. Nigerian Context
Example: A child bought a biscuit for ₦10 and a sweet for ₦
5. They calculated the total cost as ₦
1
5. Cross-check:**
1. Add in a different order: ₦5 + ₦10 = ₦15. (Correct)
2. Use subtraction: ₦15 - ₦5 = ₦10 (the cost of biscuit). (Correct)
3. Re-add: Count 10 fingers, then 5 more fingers (11, 12, 13, 14, 15). (Correct) * This confirms the total cost is indeed ₦
1
5. This topic covers two main concepts: finding missing numbers in addition statements and cross-checking addition calculations. A. Finding Missing Numbers in Addition Statements This concept introduces students to the idea that sometimes one of the numbers being added is unknown, but the total is known. Students need to determine the unknown addend.
Core Idea: If we know the total (sum) and one part, we can find the other part by "taking away" the known part from the total. This introduces the inverse relationship between addition and subtraction at a foundational level.
Example 1: Missing Second Addend Statement: 3 + ? = 5 Explanation: "We have 3 items, and we need to reach a total of 5 items. How many more items do we need to add?" Step-by-step reasoning:
1. Start with the known addend: 3.
2. Count up from 3 until the total (5) is reached: "3... (count 4, 5)".
3. Count how many steps were taken: 2 steps (for 4 and 5).
4. Therefore, the missing number is 2.
5. Check: 3 + 2 = 5. (Correct)
Alternatively (using subtraction concept):
1. Think of it as having 5 items and taking away 3 of them. 2. 5 - 3 = 2.
3. So, 3 + 2 =
5. Example 2: Missing First Addend Statement: ? + 2 = 7 Explanation: "We added 2 items to some other items, and now we have a total of 7 items. How many items did we start with?" Step-by-step reasoning:
1. Start with the total: 7.
2. Count back the known addend (2) from the total: "7... (count 6, 5)".
3. The number reached after counting back 2 steps is 5.
4. Therefore, the missing number is 5.
5. Check: 5 + 2 = 7. (Correct)
Alternatively (using subtraction concept):
1. Think of it as having 7 items and taking away 2 of them. 2. 7 - 2 = 5.
3. So, 5 + 2 =
7. Nigerian Context
Example: A trader had 4 oranges. A customer bought some more oranges from her, and now she has 9 oranges in total. How many oranges did the customer buy?
Statement: 4 + ? = 9 Solution: Start at 4, count up to 9 (5, 6, 7, 8, 9). That's 5 steps.
Answer: The customer bought 5 oranges. B. Cross-checking the Correctness of Addition This skill teaches students to verify their own answers, which is crucial for accuracy and developing independent learning habits.
Core Idea: There are different ways to check if an addition sum is correct. This reinforces the understanding of number relationships and properties of addition.
Method 1: Adding in a Different Order (Commutative Property)
Explanation: The order in which numbers are added does not change the sum.
Example: A student calculates 4 + 3 =
7. To cross-check, they can calculate 3 + 4. 3 + 4 =
7. Since both sums give 7, the original answer is likely correct.
Method 2: Using the Inverse Operation (Subtraction)
Explanation: If `addend1 + addend2 = sum`, then `sum - addend1 = addend2` (and `sum - addend2 = addend1`).
Example: A student calculates 6 + 2 =
8. To cross-check, they can subtract one of the addends from the sum.
Check 1: 8 - 2 = 6. (Since 6 is the other addend, the addition is correct).
Check 2: 8 - 6 = 2. (Since 2 is the other addend, the addition is correct).
Method 3: Re-adding Carefully Explanation: Simply performing the addition again slowly and carefully, perhaps using a different counting strategy (e.g., starting from the larger number).
Example: A student calculates 5 + 3 =
8. To cross-check, they can re-count: "Start with 5, then count 3 more: 6, 7, 8." The answer is still
8. Nigerian Context
Example: A child bought a biscuit for ₦10 and a sweet for ₦
5. They calculated the total cost as ₦15. * Cross-check:
1. Add in a different order: ₦5 + ₦10 = ₦
1
5. Teacher Activities: Review Previous Learning (5 minutes): Begin by reviewing simple addition sums (e.g., 4 + 3, 5 + 2) using manipulatives like bottle tops, seeds, or counting sticks. Ask students to show the sum and state the answer. Introduction to Missing Numbers (10 minutes): Tell a simple story: "Aisha had 3 mangoes. Her mother gave her some more mangoes. Now Aisha has 5 mangoes in total. How many mangoes did her mother give her?" Write the problem on the board as a number sentence: 3 + ? =
5. Use manipulatives to demonstrate: Place 3 items. Count out 5 items separately.
Ask: "How many more do we need to add to 3 to get to 5?" Guide students to count on from 3 to 5 (4, 5) and identify the 2 missing items.
Repeat with another example: ? + 4 =
7. Explain that the missing number can be at the beginning. Use the "count back" or "take away" strategy with manipulatives (start with 7, remove 4, see what's left). Introduction to Cross-checking (10 minutes): Present an addition sum: "Let's say Chidi calculated 6 + 3 and got
9. How can Chidi be sure his answer is correct?" Method 1 (Commutative Property): Demonstrate by adding 3 +
6. Show that the answer is still
9. Explain that changing the order of numbers does not change the total. Method 2 (Inverse Operation - Subtraction): Demonstrate by taking the sum (9) and subtracting one of the original numbers (e.g., 9 - 3 = 6). Explain that if you get the other number, then the addition was correct.
Method 3 (Re-adding): Ask students to re-count using their fingers or manipulatives for 6 + 3 to confirm
9. Guided Practice (10 minutes): Present a few more examples for missing numbers and cross-checking on the board. Guide students step-by-step using questions and prompts.
For missing numbers: "If Nkechi has ₦5 and needs ₦8 to buy 'boli', how much more money does she need?" (5 + ? = 8)
For cross-checking: "If Emeka says 7 + 2 = 9, how can he check if he is right?" Student Activities: Manipulative Engagement: Students will use their personal sets of manipulatives (e.g., counting sticks, bottle tops, pebbles) to represent and solve missing number problems posed by the teacher.
Oral Participation: Students will actively participate in class discussions, answering questions and explaining their reasoning for finding missing numbers or checking sums.
Pair Work: Students will work in pairs to solve given missing number problems and practice cross-checking each other's addition calculations. They can use slates or small whiteboards for this.
Board Work: A few students may be called to the board to demonstrate their solutions or cross-checking methods to the class.
Story Problem Solving: Students will listen to and attempt to solve simple word problems involving missing numbers and cross-checking, relating them to everyday scenarios. The teacher should guide students through these problems, encouraging them to use manipulatives and explain their thinking.
Question: Find the missing number: 6 + ? = 9 Worked Solution: Start with
6. Count up to 9: 7, 8,
9. The numbers counted are 3 (7, 8, 9 represents 3 counts). So, the missing number is
3. Check: 6 + 3 = 9. (Correct)
Commentary: This helps students conceptualize finding the difference between the known addend and the sum.
Question: Find the missing number: ? + 4 = 7 Worked Solution: Start with the total,
7. Count back 4 from 7: 6, 5, 4,
3. The number reached after counting back 4 steps is
3. So, the missing number is
3. Check: 3 + 4 = 7. (Correct)
Commentary: This reinforces the idea that subtraction can be used to find a missing addend, regardless of its position.
Question: Okoro counted 5 goats in the pen. Later, his father bought 3 more goats. Okoro added them and said there are now 9 goats. How can Okoro check if his answer is correct?
Worked Solution: Okoro's sum: 5 + 3 =
9. Method 1 (Commutative Property): He can add them in a different order: 3 +
5. Count 3, then add 5 more: 4, 5, 6, 7,
8. So, 3 + 5 =
8. Since 8 is not 9, Okoro's initial answer of 9 is incorrect.
Method 2 (Inverse Operation): He can take his sum (9) and subtract one of the original numbers (3). 9 - 3 =
6. Since 6 is not 5 (the other original number), his answer is incorrect.
Commentary: This guides students to apply cross-checking methods and identify errors. The correct answer for 5 + 3 is
8. Question: If a tailor used 7 buttons on a shirt and 2 buttons on a pair of shorts, he said he used 9 buttons in total.
Cross-check his sum: 7 + 2 =
9. Worked Solution: Original sum: 7 + 2 =
9. Cross-check using Method 1 (Commutative Property): Add in reverse order: 2 +
7. Count 2, then add 7 more: 3, 4, 5, 6, 7, 8,
9. Result: 2 + 7 =
9. Conclusion: Since the result (9) matches the original sum, the tailor's addition is correct.
Commentary: This shows a correct cross-check and confirms the accuracy of an addition statement.
Market Transactions (Economy): Scenario: A child goes to the market with their parent. They buy tomatoes for ₦50 and pepper for ₦
3
0. The child can add 50 + 30 = 80 to find the total cost. They can then cross-check by adding 30 + 50 or by checking if 80 - 30 =
5
0. This ensures they pay the correct amount or receive the correct change. The concept of missing numbers can be applied if they need a certain amount for an item and have only part of it (e.g., "I have ₦40, but the 'akara' costs ₦
7
0. How much more do I need?").
Counting and Sharing (Community/Culture): Scenario: During family gatherings or school events, children often help count items like 'chin-chin' or 'puff-puff' for distribution. If there are 8 plates, and they put 2 'puff-puff' on each, they can add 2+2+2+2+2+2+2+2 to find the total or use multiplication later. For this topic, if 3 children have received snacks, and 5 more need to receive, the total number of children who got snacks is 3 +
5. They can cross-check this sum to ensure no child is left out or mistakenly given too many. The missing number concept can be used if they need to share 10 oranges among 5 children, and 3 children have already received some; how many more need to be shared to reach the total?
Sports and Games (Culture/Recreation): Scenario: In local games like 'ayo' or simple counting games, children often need to add scores or items collected. If a child scores 4 points in the first round and 3 points in the second, they add 4 + 3 =
7. They can cross-check this by adding 3 + 4 to confirm their total score. This promotes accuracy and fair play, crucial values in Nigerian community interactions.