Math Is Fun Forum

sumpm1

Member

Registered: 2007-03-05

Posts: 42

Help with TI-89 converting expressions

Hi. My calculator is converting expressions such as

 (100-t)^(-4)

to

1/((t-100)^(4))

This is causing problems when integrating/differentiating. Can anyone account for such a conversion.

Thanks

Last edited by sumpm1 (2008-10-02 19:17:37)

gnitsuk

Member

Registered: 2006-02-09

Posts: 121

Re: Help with TI-89 converting expressions

These formulae are just two different ways of writing the same thing, like x + y and y + x they are just alternative ways of expressing the same quantity.

Notice also that (x-y)^2 is the same as (y-x)^2. If one of x-y and y-x is -'ve so the other will be +'ve, but both will be of the same magnitude. e.g. 7-5=2 and 5-7=-2 etc. But as a -'ve times a -'ve is +'ve we see that (7-5)^2 = 2^2 = (-2)^2 = (5-7)^2

So:

sumpm1

Member

Registered: 2007-03-05

Posts: 42

Re: Help with TI-89 converting expressions

This conversion is QUITE inconvenient for differentiation/integration as the output is either not similar to the result found by standard methods (everything is written backwards ((100-t) vs (100-t)) and sometimes leads to failure to perform calculus functions on expressions. Can I prevent these conversions?

I am wondering why the calculator INSISTS on this representation.

Ricky

Moderator

Registered: 2005-12-04

Posts: 3,791

Re: Help with TI-89 converting expressions

Representation does not affect whether the calculator can or can not take derivatives.  It may make it harder for you to do so, is this what you mean?

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"In the real world, this would be a problem.  But in mathematics, we can just define a place where this problem doesn't exist.  So we'll go ahead and do that now..."

sumpm1

Member

Registered: 2007-03-05

Posts: 42

Re: Help with TI-89 converting expressions

No, the calcualator has no problem performing calculus functions on these expressions, but the result is not similar to what you would see in a text book. Then taking second derivatives, and so forth results even stranger results. So I am saying that it seems that the calc does not work the problem out as a student would or should, it makes the result ambiguous. Also, when problems like this are presented in differential equations using initial conditions, the calculator will have a problem with the (t-100) term in the denominator, and gives an error rather than solve solvable differential equations. I mean the calc works wonders, but this is th the first time I have seen it manipulate the results of an input so much.

bobbym

bumpkin

From: Bumpkinland

Registered: 2009-04-12

Posts: 109,606

Re: Help with TI-89 converting expressions

Hi sumpm1;

   Calculators and computer algebra systems solve problems using very different methods than the human way. They have their own standards for simplification, often this will look strange. The TI 92 which is the bigger, stronger brother of the TI 89 does the exact same thing on your example. Try to use your graphing calculator to augment and check your work. Never let it replace your brain.

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In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.