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Number & Operations for Teachers Copyright David & Cynthia Thomas, 2009 |
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Set Model As
with whole numbers, a set model may be used to model addition and
subtraction. In this model, negative
numbers are represented using red disks and positive numbers are represented
using blue disks. By
definition, the indicated sum -1
+ 1, modeled as Figure
2.5 models the operation -2
+ 4 as the union of two sets, one set containing two red disks and the other
four blue disks. In the union of these
sets, pairs of red and blue disks are formed and associated with the number
zero. The remaining unpaired blue
disks model the sum, 2.
+ = = -2 + 4 = 0 + 0 +
2 = 2 Figure
2.5: A set model for the indicated sum -2 + 4 As
with whole numbers, a related fact may be asked in the form of the inverse
operation: 2 – 4 = -2. Figure 2.6 models this operation. Notice that two “zeros”, each in the
form
- = - = 2
- 4
= 0 + 0 +
2 - 4 = -2 Figure
2.6: A set model for the indicated difference 2 – 4
= -2
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