Associative law Lets play a game Find the

Associative law

Let’s play a game. Find the sum of three numbers in least time. （1）70、50、30 （2）32、40、68 （3）100、150、850 Add up to the tens 、hundreds or thousands

Task 1 The dates below are school open days where the whole class sells fruit juice to raise money for charity. Sales Date Nov. 1 st Nov. 2 nd Nov. 3 rd Apple juice 63 55 45 （tin） Q: How many tins of fruit juice were sold on these dates in total?

Task 1 63＋55＋45 =（63＋55）＋45 = 118＋45 = 163 63＋55＋45 =63＋（55＋45） =63＋100 =163 What is the same and what is different between the two number sentences? （63＋55）＋45=63＋（55＋45）

Task 1 27＋36＋64 =（27＋36）＋64 = 63＋64 = 127 27＋36＋64 =27＋（36＋64） =27＋100 =127 （27+36）+64=27+（36＋64）

Task 1 （63＋55）＋45=63＋（55＋45） （27+36）+64=27+（36＋64） Who can create a similar question?

Task 1 To add three numbers 1) Add the first two numbers 2) Now add the third number. Or 1) Add the last two numbers 2) Now add the first number. Their sum remains the same. associative law of addition

Task 1 Algebraic expression: (a + b) + c = a + (b + c)

Follow-up exercises Fill in the blanks by using the associative law of addition （33＋16）＋84= 33 ＋（16＋ 84 ） （168＋24）＋76= 168 ＋（ 24 ＋ 76 ） （25＋ 28 ）＋72= 25 ＋（28＋72） （a＋ b ）＋c=a＋（b＋ c ）

Task 2 Danny's father bought 3 boxes of juice. There are 25 cans per box and each can costs £ 4. How much did his father pay in total? 3× 25 × 4

Task 2 3× 25× 4 =（3× 25）× 4 = 75× 4 = 300 3× 25× 4 =3×（25× 4） =3× 100 =300 Which method is better? （3× 25）× 4= 3×（25× 4）

Task 2 26× 8× 125 =（26× 8）× 125 = 208× 125 = 26000 26× 8× 125 =26×（8× 125） =26× 1000 =26000 （26× 8）× 125= 26×（8× 125） （□×□）×□= □×（□×□）

Task 2 （3× 25）× 4= 3×（25× 4） （26× 8）× 125= 26×（8× 125） Who can create a similar question?

Task 2 To multiply three numbers 1) Multiply the first two numbers 2) Now multiply by the third number. Or 1) Multiply the last two numbers 2) Now multiply by the first number. Their product remains the same. associative law of multiplication

Task 2 Algebraic expression: (a × b)×c = a×(b × c)

Follow-up exercises Fill in the blanks by using the associative law of addition 36× _____ 71）× 26 36× 71× 26=（ ____ 95× ____83 57× 95× 83=57×（ ____ ） ●×▲×★=___●×（▲× __ ）★ ● × ▲）× ____ ★ =（ ___

(a ＋ b) ＋ c = a ＋ (b ＋ c) (a × b) × c = a × (b × c) Compare with the associative law of addition and multiplication，what do you find? Øthree numbers Øcalculate from left to right in turn Øcalculate the last two numbers first Øthe answer remain the same

Exercise

1、Match the number sentences (1) a×（b×c） A. 24＋（42＋58） (2) 76＋18＋22 B. 76＋（18＋22） (3) 24＋42＋58 C. 67×（125× 8） (4)（67× 125）× 8 D. （a×b）×c

2、Which number sentence conforms to the associative law? (1) a×(b×c)=(a×b)×c √ (2) 15＋(7＋3)=(15＋2)＋3 × (3) (23+41)+72+28=23+41+(72+28) √

3、Fill in the blanks by using the associative law 66 ） 25 34 (1) 25＋34＋66=___＋（ ___＋____ 25 × ____ 40）× 78 (2) 25× 40× 78=（ ____ 44 56 (3) 56＋72＋44=(___＋ ___)＋72 (4) 75× 8× 2× 125= 2 ）×（ ___8 × ___125 75 × ___ （ ___ ）

4、 Solve by making the question simpler 92＋89＋11 39× 5× 2 =92+（89+11） =39×（5× 2） =92+100 =39× 10 =192 =390 85＋41＋15＋59 = 85+15+41+59 =（85+15）+（41+59） = 100+100 = 200 125× 17× 8 =125× 8× 17 =1000× 17 =17000 64× 125 =8× 8× 128 =8×（8× 125） =8× 1000 =8000

Summary What have you learned today？

Using your brain: John and Lucy are calculating 98 ＋ 9998 ＋ 2 × 3, John said the correct answer right away. Can you？