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Re: Addition and subtraction strategies
Posted:
Jul 8, 2002 11:20 AM
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Another way might be this: when we are looking at an arithmetic
expression, it is a phrase, not a whole sentence. Addition phrases are
always about two parts that we want to combine to find a whole.
Subtraction phrases are always about a whole, and some part we wish to
take away (until we subtract a larger number from a smaller one, in
which case we are going to have to reconsider!). So, when we add, we
may consider a related phrase, say by adding some to one part, and by
taking away the same from the other part, and we will still end up
getting to find the same whole by putting parts together. But, in a
phrase to find a difference between a whole and one of its parts, we
may well find that a different whole (larger by x) is just as far away
from a larger one of its parts (larger by exactly x also) and so find
our own difference by solving this related problem. The problem may
be in stating all of this to the kids.
I suggest that you ask them to make two boxes for the parts and relate
them to a third box, named the "whole". THen, ask them where the
parts and whole might go to fit the addition problem. Next, erase
that, or write in parallel, and ask them to use the set of three boxes
again, but show the subtraction situation this time. The notion of
parts and whole may help. I like your use of the number line very
much however!
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