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Topic: el fractions etc
Replies: 14   Last Post: Aug 10, 1995 8:05 AM

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Rex Boggs

Posts: 12
Registered: 12/6/04
re: el fractions etc
Posted: Aug 9, 1995 5:48 AM
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Hi Tonna and NCTM

>Karen, typing fractions on this machine will be a challenge but might be
>worth it if others can follow my way of showing students why the algorithm
>is to invert the divisor and multiply to divide fractions. Here goes: etc


I use a similar method to teach division of fractions in grade 8. The kids
need the same background as Tonna stated, as well as having multiplied
enough fractions by their recipricals to know that the product is 1.

> 1
> 1 3 ___
> ___ divided by ___ 2
> 2 5 ...is the same as _______
>
> 3
> ___
> 5
>
> ...a complicated
> fraction!


Here I multiply by 5/3 (the reciprical of the denominator) instead of just
5, as Tonna does...

1 5 1 5 1 5
___ x ___ ___ x ___ ___ x ___
2 3 2 3 2 3
____________ = ____________ =
3 5
___ x ___ 1 ...as the 1
5 3 in the denominator
drops out

And thats it -

1 3 1 5
___ divided by ___ becomes ___ x ___
2 5 2 3

the reciprocal-and-multiply algorithm we all use.

A further issue, while we are on the topic, is _why_ do we divide fractions?
(I'll use '/' for the divide symbol) I can see a reason for 1/4 / 2, as
this could arise from the question 'How much of a whole pizza does each
person get if 2 people share the remaining 1/4 of a pizza?'.

But what question can you pose where the necessary operation is
dividing, say, 1/4 by 3/5? I used to think there weren't any such
questions, until I read a good response last year (somewhere on the Net..)

Think of division of fractions another way -
first think of 3 divide 6 = 1/2 as meaning that 3 is 1/2 of 6. Also,
1/4 divide 2 = 1/8 means that 1/4 is 1/8 of 2, So 1/4 divide 3/5
becomes ' 1/4 is what part of 3/5?' and the answer is that 1/4 is 5/12 of
3/5.

I haven't come across anything better than that.

Cheers

Rex

Rex Boggs | High above the hushed crowd,
rex@cq-pan.cqu.edu.au | Rex tried to remain focused.
| Still, he couldn't shake one
Glenmore High School | nagging thought: He was an
Rockhampton, Quensland | old dog and this was a new
Australia | trick - Gary Larson






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