Transcript here. - Holy Rosary Website

Multiplying Decimals
Type your name and send:
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To Multiply:
• Place the number with more digits on top.
• Multiply
• Count the number of decimal places (from the
right) in the question.
• Make sure that you have the same number of
decimal places in your answer.
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Example: 5.63 x 3.7
1
4 2
5.63
x 37
39 4 1
+ 168 9 0
20.8 31
.
two
one
1 1
1
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three
2.3 x 1.4
When multiplying these two numbers
together, it is easier to put
them into a grid.
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2 decimals to the
right
2.3 x 1.4
20
10
4
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For now let’s cheat and pretend that it is
not Decimals – just remember there are two
numbers to the right in the question.
The first thing to do is
partition the numbers into
tens and units.
3
2 decimals to the
right
2.3 x 1.4
10
Multiply the number above
the box with that on the left,
writing the answer inside the box,
as shown below.
20
3
200
30
What goes
here?
4
80
Next slide
12
2 decimals to the
right
2.3 x 1.4
10
The next step is to add up
the rows, putting the answer
in the boxes on the right.
20
3
200
30
230
What goes
here?
4
80
Next slide
12
92
2 decimals to the
right
2.3 x 1.4
Now add up the totals on the
right to NEARLY give the answer.
20
3
10
200
30
230
4
80
12
92
Next slide
322
What goes
here?
2 decimals to the
right
2.3 x 1.4
How many numbers to the right in
the question?
20
3
10
200
30
230
4
80
12
92
Next slide
322
2 decimals to the
right
2.3 x 1.4
How many numbers to the right in
the answer?
20
3
10
200
30
230
4
80
12
92
Next slide
.
3 22
Now you can work out multiplication
using the grid method, you will find long
multiplication easy!
This is a little different
from the grid method,
as the numbers are
set out in columns…
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5.1 x 4.2
becomes
Qwizdom question
– what would your
estimate be? Whole
numbers only using
nearest nice
number?
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51
x 42
5.1 x 4.2
The first thing to do, is
multiply the tens of the
top number with the tens
and units of the bottom
number.
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51
x 42
Remember what
we did using
the grid method?
5.1 x 4.2
50 x 40
50 x 2
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Next, multiply the
units of the top
number by the tens
and units of the
bottom number.
51
x 42
2000
100
5.1 x 4.2
50 x 40
50 x 2
1 x 40
1x 2
Next slide
51
x 42
2000
100
40
2
Don’t forget to
write down the
multiplication
next to the
answer.
5.1 x 4.2
50 x 40
50 x 2
1 x 40
1x 2
Qwizdom – what Next slide
would the final
total be?
51
x 42
2000
100
40
2
2142
The NEARLY final
step is
to add up the
column of
numbers and put
the total at
the bottom.
5.1 x 4.2
50 x 40
50 x 2
1 x 40
1x 2
Next slide
What is the actual
answer?
51
x 42
2000
100
40
2
2142
Multiplication Properties
Commutative Property of Multiplication: Factors can be multiplied
in any order.
Example: 12 x 5 = 5 x 12
Associative Property of Multiplication: Factors can be grouped in
any way.
Example: (2 x 3) x 5 = 2 x (3 x 5)
Identity Property of Multiplication: The product of any number and
one is that number.
Example: 18 x 1 = 18
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LO: To be able to multiply with decimals
A
I don’t understand this idea at all
and need to work again on this
unit of work.
B
I have some understanding but
I need more practice.
C
I understand this idea fully.
End of slide show