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Subtraction

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Models and images can we ... you can see all five lily pads, the splashes from the two ... Models teachers use to help. firstly using number beads ... – PowerPoint PPT presentation

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Title: Subtraction


1
Subtraction
2
Where does subtraction start? FOUNDATION 2
  • Taking away say how many are left when some
    are taken away, by counting how many are left.
  • Counting back say how many are left when
    some are taken away, by counting back from the
    number.
  • Counting up find out how many have been
    removed by counting up to the larger number.
  • How many more find out by counting up how
    many more will make a given number.

3
From Year 1
  • From Year 1 onwards children should continue to
    develop understanding of subtraction as
  • taking away
  • finding the difference between how many
    more/how many less?
  • They should also begin to appreciate that
    subtracting zero leaves a number unchanged.
  • Like addition written algorithms 5 3 ? etc

4
From Year 2
  • From Year 2 onwards children also need to
    appreciate that
  • 4 2 is different from 2 4 (not commutative)
  • subtraction reverses addition (the inverse)
  • From Year 3/4 children also need to understand
    that
  • when a larger number is subtracted from a
    smaller number, the answer is negative e.g. 3
    8 -5

5
Counting on
Look at the following calculations. Decide
which ones you would count on
89 11 33 19 21 - 10 109 98 88
12 2002 1993 99 29 83 70 4379
123 303 30 66 59 6712 1001
Close numbers count on
6
Models and images can we give children?
It is important that children are given the
correct images for subtraction. This is an
example of an image which isnt helpful.
7
9 - 3
8
9 - 3
9
9 - 3
10
9 - 3 6
11
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12
This is a good image as it shows the whole
picture you can see all five lily pads, the
splashes from the two frogs that have jumped in
and the remaining three frogs.
13
DRAWING A PICTURE STORY IS OFTEN VERY HELPFUL FOR
CHLDREN WHO STRUGGLE.
14
100 GRIDS
Back 1 up 2
Now lets subtract 21
15
Continual Practise
  •          Using a known addition fact - where a
    child gives an immediate response to
    facts known by heart, such as 10 - 5
    or 20 - 9
  •      Using a known fact to derive a new fact
  • 20 - 5 is 15, so 20 - 6 must be 14
  •       Using knowledge of place value
  • 70 - 30 ? 7 - 3 is 4 therefore
    answer of 40.
  • .

16
Models teachers use to help.
firstly using number beads
17
43 - 26
This calculation can be modelled two ways.
18
43 - 26
You can find the first 43 beads and then
19
BLANK NUMBER LINES
Both understandings of subtraction (take away
and difference) can be modelled on number lines.
Counting back
43 26 17
43
33
23
17
20
-3
- 3
-10
-10
X 6
10
4
3
Bridging
26
30
40
43
20
YEAR5/6
Number lines for more complex calculations
COUNTING/ BRIDGING
21
KEY SKILL - AGAIN PARTITIONING
22
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