Multiplication facts and multi-digit multiplication
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给老师： 面向 Multiplication facts and multi-digit multiplication（Primary Maths，Grade 4）的即用型课程幻灯片, 复习笔记——用在你的课堂上，或将该知识点作为学生可实时游玩的互动课堂活动来运行。
课程笔记
What is Multiplication?
- Multiplication is a quick way to add the same number many times. For example, 4 × 3 means 3 + 3 + 3 + 3 = 12.
- The × sign is called the multiplication sign. The numbers you multiply are called factors, and the answer is the product.
- You can show multiplication with an array – objects arranged in rows and columns. For example, 3 rows of 4 dots is 3 × 4 = 12.
- Multiplication can also compare amounts: “4 times as many” means multiply by 4. If you have 5 apples and your friend has 4 times as many, your friend has 5 × 4 = 20 apples.
- The order of factors does not change the product. This is the commutative property: 4 × 3 = 3 × 4 = 12.
Multiplication Facts Up to 12 × 12
- You should memorize the multiplication table (also called times table) for numbers 1 to 12. It shows all products like 7 × 8 = 56.
- The table is symmetric: 7 × 8 = 8 × 7, so you only need to learn half of it.
- Multiplying by 0 always gives 0: 0 × 9 = 0.
- Multiplying by 1 gives the same number: 1 × 6 = 6. This is the identity property.
- Multiplying by 10 is easy: just put a 0 at the end. For example, 4 × 10 = 40.
- Practice with flashcards or games to recall facts quickly and correctly.
Multiplying Three Numbers
- You can multiply three numbers together, like 2 × 3 × 4.
- The order does not matter, so you can group any two first. For example, (2 × 3) × 4 = 6 × 4 = 24, and 2 × (3 × 4) = 2 × 12 = 24.
- This is called the associative property of multiplication.
- When you see a multiplication with three factors, just multiply two at a time.
Multiplying by 10 and 100
- When you multiply a number by 10, each digit moves one place to the left. For example, 35 × 10 = 350.
- When you multiply by 100, each digit moves two places to the left. For example, 35 × 100 = 3500.
- You can think of it as adding zeros: × 10 adds one zero, × 100 adds two zeros.
- This works because our number system is base-ten – each place value is ten times bigger than the one to its right.
- Dividing by 10 or 100 moves digits to the right: 350 ÷ 10 = 35, and 3500 ÷ 100 = 35.
Multiplying a Two-Digit or Three-Digit Number by a One-Digit Number
- Use the formal written method (also called the column method) to multiply bigger numbers.
- Write the one-digit number under the ones place of the bigger number. For example, to do 23 × 4, write 4 under the 3.
- Multiply the ones digit first: 3 × 4 = 12. Write 2 in the ones place and carry the 1 to the tens column.
- Then multiply the tens digit: 2 × 4 = 8, and add the carried 1 to get 9. So 23 × 4 = 92.
- For three-digit numbers, do the same: multiply ones, then tens, then hundreds, carrying as needed. For example, 123 × 3: 3×3=9 (ones), 2×3=6 (tens), 1×3=3 (hundreds), so 123 × 3 = 369.
- Always check your answer by estimating or using repeated addition.
Patterns in the Multiplication Table
- The multiplication table has patterns that help you memorize facts.
- For multiples of 2, 4, 6, and 8, the ones digits follow a repeating pattern. For example, multiples of 2 end in 2, 4, 6, 8, 0.
- Multiples of 5 always end in 0 or 5.
- Multiples of 9 have a neat trick: the digits add up to 9. For example, 9 × 3 = 27, and 2 + 7 = 9.
- Multiples of 10 always end in 0.
- You can use these patterns to check your answers or to find a fact you forgot.
Solving Word Problems with Multiplication
- Read the problem carefully and look for words like “times”, “each”, “per”, or “product” – they often mean multiply.
- If a problem says “4 times as many”, multiply the original amount by 4.
- If you have groups of equal size, multiply the number of groups by the size of each group. For example, 5 bags with 6 apples each: 5 × 6 = 30 apples.
- Draw a picture or an array to help you see the problem.
- Write a number sentence and solve it. Then check that your answer makes sense.
Why Memorize Multiplication Facts?
- Knowing your times tables up to 12 × 12 makes math faster and easier.
- It helps you with bigger multiplication, division, fractions, and many real-life problems.
- Memorizing facts is like learning a new language – practice a little every day.
- Use tricks, songs, or patterns to make memorizing fun.
- Once you know the facts, you can solve problems without counting on your fingers.
幻灯片
练习题
免费预览——63 题中的 8 题。注册即可查看全部。
1.What is 7 × 8?
Easy- A56
- B54
- C48
- D64
2.Which of these is the same as 9 × 6?
Easy- A6 × 9
- B9 + 6
- C9 × 9
- D6 × 6
3.The product of any number and 1 is that number itself.
EasyTrue or false?
4.What is 23 × 4?
Medium- A92
- B82
- C102
- D72
5.Which of these is the correct product of 345 × 6?
Medium- A2070
- B207
- C20700
- D20070
6.Which of the following are equal to 3 × 4 × 2? (select all that apply)
Medium- A24
- B4 × 3 × 2
- C2 × 4 × 3
- D3 × 4
- E12 × 2
7.Match each multiplication fact to its product.
Easy- 6 × 7
- 8 × 9
- 5 × 12
- 42
- 72
- 60
8.Put these numbers in order from smallest to largest.
Medium- 3 × 5
- 4 × 4
- 2 × 9
- 7 × 2
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