Maths task 20.11.2020LO: To check calculations using the inverse method.

Commutative law!

RECAP from yesterdayHow could we solve this question?

32 + 28 + 19Answer this as quickly as you can!

How can we check our answers without a calculator?

How can we check our answers without a calculator? By using inverse!

LO: To check calculations using the inverse methodWhat is inverse?

The inverse of addition is… subtraction!

For example 32 + 28 + 19 = 79. The inverse of this number sentence is 79 – 60 = 19

When you are checking the inverse with 3 numbers you need to take away the sum of two numbers to get the other number as your answer.

7 9-1 96 0

3 28

5 1

2

Solve this question using either mental maths or written method and use inverse (subtraction) to check

your answer!

22 + 38 + 16

Solve this question using either mental maths or written method and use inverse (subtraction) to check your answer!

22 + 38 + 16 = 76Mental maths jottings

20 + 30 + 10 = 60

2 + 8 = 10

60 + 10 = 70

70 + 6 = 76

The written method – column addition

3 8

2 2

1 6

+

6

1

7

Using inverse (subtraction) to check your answer!

22 + 38 + 16 = 76We always start with the biggest number – commutative law!

We need to take away the sum of two numbers from our largest number 22 + 38 = 60

7 6

6 0-

61

What do the angles in a triangle add up to?

What do the angles in a triangle add up to?180!

40

60

80

What do the angles in a triangle add up to?180!

b

a

c

a = 33

b = ?

c = 69

Use your knowledge of adding strategies and inverse to find the missing angle in

the triangle

What do the angles in a triangle add up to?180!

b

a

c

a = 33

b = ?

c = 69

Use your knowledge of adding strategies and inverse to find the

missing angle in the triangle

6 9

3 3+

2

1

1 0

1 8 0

1 0 2

__ 7 1

870

b = 78

Your turn!Solve each of these questions like you did yesterday but show that you

can check your answers using inverse (subtraction)

1 star 2 star

1. 27 + 13 + 11

2. 32 + 18 + 10

3. 45 + 25 + 13

4. 21 + 49 + 17

5. 36 + 14 + 18

1. 48 + 22 + 19

2. 66 + 34 + 21

3. 67 + 43 + 15

4. 103 + 47 + 23

5. 245 + 135 + 47