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Error on the TI-89 Titanium

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  1. Nov 27, 2013 #1
    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?
     
  2. jcsd
  3. Nov 27, 2013 #2

    Borek

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    Is -22 intended to mean -(22) or (-2)2?
     
  4. Nov 27, 2013 #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
     
  5. Nov 28, 2013 #4
    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]
     
  6. Nov 28, 2013 #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
     
  7. Nov 28, 2013 #6

    Mark44

    Staff: Mentor

    The above is a very silly use of parentheses.

    (-2)2 should evaluate to +4.
    -22 should evaluate to -4.
    The trouble with this notation, above, is that the - sign is not binding to anything.
     
  8. Nov 28, 2013 #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: Nov 28, 2013
  9. Nov 29, 2013 #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.
     
  10. Nov 29, 2013 #9

    Mark44

    Staff: Mentor

    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.
     
  11. Nov 29, 2013 #10
    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:
     
  12. Nov 29, 2013 #11

    Borek

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    Staff: Mentor

    Now that I look at the picture of my TI, I think it is time to clean it :blushing:
     
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