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Key
Idea
There is more than one way to add and subtract
mentally. |
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| Vocabulary |
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Commutative Property of Addition
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Associative Property of Addition |
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compatible numbers |
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compensation |
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 | Adding
and Subtracting
Mentally |
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Why add? |
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How much money did Marcus Shoes make in
all? |
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$300 + $1,800 + $4,700 = Total
money Marcus Shoes made |
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How
can you add mentally? |
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You can use the Commutative
and Associative Properties to
add mentally. For example, when you add three numbers,
you can start with any two numbers first. |
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Commutative
Property
of Addition |
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Associative
Property
of Addition |
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You
can add two
numbers in any order. |
| 12
+ 25 = 25 + 12 |
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You
can change the
grouping of addends. |
| 12
+ (20 + 5) = (12 + 20) + 5 |
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You can also use compatible
numbers, which
are numbers that are easy to compute mentally.
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With compensation,
you adjust one number to
make computations easier
and compensate by
changing the other number. For
example, to find
39 + 58, you could add 1 to 39
and then
subtract 1 from 58. You have really just
added 0
to 39 + 58. Remember the
Identity Property of
Addition tells you
that the sum of any number
and 0 is that number. |
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