Error on the TI-89 Titanium

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  • #1
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I was working with a much longer equation and receiving a result I didn't expect, and finally narrowed it down to the following section:

[itex]2(-2^3)-3(-2^2)[/itex]

2(-2^3) is correctly calculated as -16 independently, and 3(-2^2) is correctly calculated as 12 (again, independently) for a difference of -28, but my calculator is returning -4.

No matter how I enter the equation, the only way I obtain a correct result is by first defining a variable as -2 and then substituting that variable in for -2 in the equation.

Is this problem repeated on anyone else's unit? And if you've encountered this sort of problem before, is there a way to correct it?
 

Answers and Replies

  • #2
Borek
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Is -22 intended to mean -(22) or (-2)2?
 
  • #3
lurflurf
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I will use (-) for the negative operator

I hope 3((-)2^2) is not calculated as 12 that would be very wrong

3((-)2^2)=-12 due to operator precedence

2((-)2^3)-3((-)2^2)=2((-)(2^3))-3((-)(2^2))=-4

perhaps you had intended to write

2(((-)2)^3)-3(((-)2)^2)=-28
 
  • #4
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There are a number of different ways to enter the problem to achieve the intended result. I have tried a few variations, all with the same results.

[itex](2*((-2)^3))-(3*((-2)^2))=-4[/itex]
[itex](2*((z)^3))-(3*((z)^2))=-28[/itex]

This is a piece of the derivative of a function, and -2 is one of the zeros of the derivative signifying a local maximum in the function. The intent is to solve the equation when z=-2 (among other values)

Incidentally, entering the equation in the calculator as I originally did produces the same results; the intent is not to find -(2^3) (which is -8) but (-2^3) (which is also -8, but for a different reason).

[itex](2*-2^3)-(3*-2^2)=-4[/itex]
[itex](2*z^3)-(3*z^2)=-28[/itex]
 
  • #5
lurflurf
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^Of those four only the first one is surprising. If that input gives that output I am quite confused.
What happens if you enter
2((0-2)^3)-3((0-2)^2)
?
I do not have a ti89 handy
I do know that the manual gives the example
((-1)2)^2=4
(-)2^2=-4
 
  • #6
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I will use (-) for the negative operator

I hope 3((-)2^2) is not calculated as 12 that would be very wrong
The above is a very silly use of parentheses.

(-2)2 should evaluate to +4.
-22 should evaluate to -4.
3((-)2^2)=-12 due to operator precedence
The trouble with this notation, above, is that the - sign is not binding to anything.
2((-)2^3)-3((-)2^2)=2((-)(2^3))-3((-)(2^2))=-4

perhaps you had intended to write

2(((-)2)^3)-3(((-)2)^2)=-28
 
  • #7
lurflurf
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^It is not silly, it is to distinguish between the unary and binary operators. It is the same notation used on the calculator keypad. For you special
3(-<<<the unary one>>>2^2)

to quote the manual

$$\text{Important: Use }\bbox[3px,border:2px solid black]{\phantom( - \phantom)}\text{ for subtraction and use }\bbox[3px,border:2px solid black]{(-)}\text{ for negation.}$$
 
Last edited:
  • #8
Borek
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Mark, not sure if you know it, but TI-89 has two different minus keys.

TI-89_minus_keys.jpg


The one with "(-)" is an unary "change sign" operator, the other is a binary "minus". Hence the "(-)" and "-" notation is just reflecting the reality.
 
  • #9
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Mark, not sure if you know it, but TI-89 has two different minus keys.
No, I didn't know that. That notation seems to be fairly new in calculators. Calculators have been distinguishing between the unary minus and binary subtraction operator for a long time, but using +- for the unary operation and - for subtraction.

I didn't realize that lurflurf was using (-) to mimic that key on the TI-89.
The one with "(-)" is an unary "change sign" operator, the other is a binary "minus". Hence the "(-)" and "-" notation is just reflecting the reality.
 
  • #10
pbuk
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No, I didn't know that. That notation seems to be fairly new in calculators. Calculators have been distinguishing between the unary minus and binary subtraction operator for a long time, but using +- for the unary operation and - for subtraction.

I didn't realize that lurflurf was using (-) to mimic that key on the TI-89.

The [(-)] key doesn't mimic a [+/-] key, it correctly implements the operation of negation in normal mathementical notation which is to negate the following argument, whereas the normal implementation of a [+/-] key is to negate the argument currently displayed.

Note that page 943(!) of the manual states:
To enter a negative number, press [(-)] followed by the number. Post operations and exponentiation are performed before negation. For example, the result of -x2 is a negative number, and -92 = -81.
 
  • #11
Borek
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Now that I look at the picture of my TI, I think it is time to clean it :blushing:
 

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