-10 = 5.5a is false? Why?

[Reversearp]

Hi all,

I searched to see if this was answered already but didn't find anything.  Anyway, this visual: http://screencast.com/t/VKojIEzx5c explains it better than words, but essentially I am calculating -10/5.5 and storing the result as a variable (a).  Then typing -10 = 5.5a returns false  and a message about using an approximate arithmetic.

Even storing (-10/5.5) as "a" and then doing 10 = 5.5a returns false.  Why is that?

Thanks!

[Michael Houston]

Try this: -10/5 store as (a).  Then type -10 = 5*a and see what happens...

mike

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Date: Wed, 25 Jan 2012 08:14:48 -0800
From: abie...@gmail.com
To: tins...@googlegroups.com
CC: hust...@gmail.com
Subject: [tinspire] -10 = 5.5a is false? Why?

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[Sean Bird]

When a decimal is included, the solution will automatically be given as an approximation. If there was a way to ask it something was correct withing a certain tolerance, say for example +/-0.000001, then it would say true and that would be good enough for much of life.

But as far as mathematics goes, we couldn't really say it is true. See the screen shots for an alternative method so you can avoid the decimal and get the true.

- Sean Bird

[Reversearp]

Ok. Thanks.  What about this:  If I compute sqrt(2) and store it as a decimal (say, d) and then I test d * d = 2 or d * sqrt(2) = 2, the calculator returns true.  Why isn't that the same situation as the 10/5.5 example?

[Reversearp]

Yeah, no that works.  I'm ok with that.  It's the test that bothered me.  If 5*a returns -10 then I expected 5*a = -10 to return true.

[Travis Bower]

Try searching the archives...approx or square root of 2 or etc.
I asked a similar question awhile ago [root 2 worked and root 3 did not or something like that]  Nelson provided insight about 14-place rounding etc.
Maybe that will help.

On Wed, Jan 25, 2012 at 1:31 PM, Reversearp <abie...@gmail.com> wrote:

Yeah, no that works.  I'm ok with that.  It's the test that bothered me.  If 5*a returns -10 then I expected 5*a = -10 to return true.

--

[PyramidPower]

If you want to use a tolerance to allow for the decimal approximation
then use the abs function. for example suppose a:=sqrt(2) and
b:=1.4142 and you want to treat a and b being equal,
then the test will be abs(a-b)<0.0001 will return true

On Jan 26, 8:38 am, Travis Bower <tbo...@dphs.org> wrote:
> Try searching the archives...approx or square root of 2 or etc.
> I asked a similar question awhile ago [root 2 worked and root 3 did not or
> something like that]  Nelson provided insight about 14-place rounding etc.
> Maybe that will help.
>
>
>
> On Wed, Jan 25, 2012 at 1:31 PM, Reversearp <abien...@gmail.com> wrote:
> > Yeah, no that works.  I'm ok with that.  It's the test that bothered me.

> >  If 5*a returns -10 then I expected 5*a = -10 to return *true*.

>
> > --
> > To post to this group, send email to tins...@googlegroups.com
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> > For more options, visit this group at
> >http://groups.google.com.au/group/tinspire?hl=en-GB?hl=en-GB
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>

> >https://sites.google.com/site/tinspiregroup/classroom-news/welcome-ab...