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Topic: Is (t^2-9)/(t-3) defined at t=3?
Replies: 166   Last Post: Oct 30, 2013 9:41 AM

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Hetware

Posts: 148
Registered: 4/13/13
Re: Is (t^2-9)/(t-3) defined at t=3?
Posted: Sep 28, 2013 5:20 PM
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On 9/28/2013 4:24 PM, Richard Tobin wrote:
> In article <9cSdnYGhH7CMpNrPnZ2dnUVZ_sCdnZ2d@megapath.net>,
> Hetware <hattons@speakyeasy.net> wrote:
>

>> So the answer is consensus among mathematicians holds that F(t) = (t^2 -
>> 9)/(t - 3) is undefined at t=3?

>
> Yes.
>

>> Perhaps what I should have said at the
>> outset is something along the lines of: on any given day, if I'm setting
>> up an equation in physics, and produce an expression such as F(t) = (t^2
>> - 9)/(t - 3), I treat it as t+3, and do not expect any adverse
>> consequence from doing so.

>
> Your simplification is not valid for t=3. If there is a real
> physical interpretation, perhaps you can derive the formula t+3
> without going through the intermediate form (t^2-9)/(t-3). Or
> consider the special case t=3 to show that the result is indeed
> t+3 in that case too.
>
> In fact, I would be interested to see a physical problem where you
> can't do that.
>
> -- Richard
>


I believe most mathematicians solving for x as a function of t given

t^2 - 9 = x (t - 3)

would not hesitate to factor the left hand side and divide both sides by
t - 3 without treating t = 3 as a special case. Doing so repeats the
sin of dividing by zero twice. We can certainly solve

t^2 - 9 = 6 (t - 3)

without dividing by zero which seems to justify our implied sin.


Date Subject Author
9/28/13
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Hetware
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Michael F. Stemper
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Peter Percival
9/29/13
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quasi
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Hetware
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Richard Tobin
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Hetware
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Hetware
10/6/13
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Hetware
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Hetware
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fom
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Richard Tobin
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10/16/13
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10/19/13
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10/20/13
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Virgil
10/18/13
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Hetware
10/19/13
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Peter Percival
10/19/13
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fom
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Peter Percival
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Hetware
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Peter Percival
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Hetware
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10/19/13
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Hetware
10/19/13
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