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