Index: (This is a new sheet as of 6/14/07, so there are only a few FAQs.  Hardly any of of my students have TI-89 calculators, so if you have any questions or problems that you've dealt with, send them to me, and I'll include them on this page.)

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(1) How do I make the screen brighter on a TI-89?
(2)  During the summer when I'm not in school, should I remove the AAA batteries?
(3)  Why will I loose my programs if I remove all batteries.
(4)  Do I really need to change the batteries on my TI-89 when I upgrade the operating system?
(5)  I found a good deal at a pawn shop.  Can I tell how old the calculator is?
(6)  Do you approve of buying a calculator from a pawn shop.
(7)  When I try to factor with my TI-89 Titanium, I get a number rather than a factor.  Why?
(8)  When I try to expand with my TI-89 Titanium, I get a number rather than a factor.  Why?
(9)  When I try to graph with my TI-89 Titanium, I get a dimension error message.  Why?
(10)  I get an error message on my TI-89 Titanium when doing IRR.  Why?
(11)  When I multiply 360*$1258.59, I get 453092.  Why did it cut off the decimal.
(12) 
(13) 
(14) 
 

Answers:

(1) How do I make the screen brighter on a TI-89?
Press and hold the shift for the green ( the key with the diamond shape on it) ; then tap the minus sign to decrease the contrast or the plus sign to increase it. 
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(2) During the summer when I'm not in school, should I remove the AAA batteries?
 No, leave them in.  When you remove the AAA batteries, the power for the RAM is supplied by the small circular battery (SR44SW or 303.)  If you leave the AAA batteries out too long, the circular battery will be discharged and you will lose any programs that you have in RAM.
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(3)  Why will I loose my programs if I remove all batteries.
 Well, I'm not exactly sure why you'd want to remove all of the batteries, but if you do, you'll lose anything that is stored in RAM.  Programs and applications that have been stored in flash ROM, archived programs, will not be lost.   But any un-archived programs will be lost since they are stored in volatile memory.  It's just like when you turn off your computer, you lose anything that hasn't been stored on a hard drive.
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(4)  Do I really need to change the batteries on my TI-83+ when I upgrade the operating system?
Oh, absolutely.  I would definitely do it. The OS is stored in flash ROM.  The way flash ROM works is that when you want to store a logic 1,  a voltage is applied that causes so-called hot electron to jump the band gap between the semiconductor and gate insulator and get trapped as charge in the gate insulator.  I don't know what type circuit TI uses to do  this, but it stands to reason that the amount of charge stored in the insulator will be proportional to the battery voltage. The greater the  trapped charge the  "harder on" the transistor will be  and, therefore, the the lower the drain-to-source resistance.  So,  some of the things you might expect form fresh batteries are the following:  
1)  The upgrade will probably go faster because the higher voltage of a new battery will charge the stray capacitances inside the circuitry faster. 
2)  Flash memory will have slightly higher logic levels stored on their cells providing a more definite logic level for the sense amplifier. That would improve  noise immunity resulting in fewer  transfer errors. 
3)  The charge on the cells might also last a little longer in case you happen to have a slightly leaky cell.
That's probably more than you ever wanted to know on that subject, so enough.  The way I do the upgrade  is to keep a set of new batteries on hand, and when I upgrade a student's calc, I put in the new batteries.  Then when the upgrade is finished, I reinstall the student's older batteries.
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(5)  I found a good deal at a pawn shop.  Can I tell how old the calculator is?
I'm not sure about that.  I don't know whether  the ID numbers contain a date code.   But you can get the approximate date from the operating system date, assuming that the calculator has never been upgraded.  To get that date, press APPs; then 3.
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(6)  Do you approve of buying a calculator from a pawn shop.
 Sorry, because of possible legal implications, I do not make specific recommendations about where to buy or which calculator to buy.  There are a couple websites that recommend against buying used calculators, but I suspect some of them may  have vested interests in those recommendations.  Here  a few possible disadvantages:
    a)  You may or not get a user manual.  On the other hand you may, and you can always download the one from TI's Website.
    b)  You probably won't get the CD, but you can download TI Connect form TI's Website.
    c)   You may not get the USB-to-Computer cable or the Unit-to-Unit cables, but you might be able to borrow them from some of
           your friends who bought new calculators if you need them.
The main advantage is that you should be able to  save big bucks if you do some comparison shopping.
    NOTE:  A couple tips about buying from pawn shops: 1)  Check the OS system date to make sure it's not really old, 2)  Make  
    sure they  have at least 7-day unlimited return policy and give the calculator a good workout during that period, 3)  pawn
    shop prices generally are not writ in stone the way retails prices are, so you may be able to negotiate the price a little.
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7)  When I try to factor with my TI-89 Titanium, I get a number rather than a factor.  Why?
First do this check:   To check that go to the Home screen and press the variable you're trying to factor, for example,
 x, y or z; then press ENTER. If anything other than the variable name is displayed, for example a number,  you'll have
 to clear the variable.
How to clear the variable:  To clear  that from the home screen, press 2ND, F1, for F6, and select New Prob.  Press
ENTER and when the "are you sure" warning comes up, press ENTER.  That should take care of it.
Discussion:  You probably inadvertently  stored in the variable you're trying to factor.  They're probably some combination
of x, y, or z. You don't want anything stored in those variables when you're doing this operation.
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(8)  When I try to expand with my TI-89 Titanium, I get a number rather than a factor.  Why?
  Same as item 7 above.
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(9)  When I try to graph with my TI-89 Titanium, I get a dimension error message.  Why?
How to correct the problem:  Go to the graph screen and scroll up beyond the y1 entries and observe all of the
plots.  If one has a check mark by it, press F4 while you have the plot number highlighted to remove the check mark.  Do that for each of the several plots.  That should take care of it.  If perchance that doesn't work, do this: From the home screen, press 2ND, F1, for F6, and select New Prob.  Press ENTER and when the "are you sure" warning comes up, press ENTER.  That should take care of it.
Discussion:  Unless you've done some rather sophisticated variable creation, the entities that have dimensions are lists and
matrices.  Now, you can graph plots and functions at the same time.  So, if you have a plot checked when your graphing a function, the calculator will try to graph that plot in addition to your function.  If the dimension of the plot is not correct
you'll get a dimension error.  A dimension mismatch occurs with a plot when you're using a plot that requires both x- and
y-values.  These will be in two different lists, for example list1 and list2.  If the number of x-values in more or less than the
number of y-values, then you'll have a mismatch.  Since you don't want to graph the plot anyway, the easiest solution is
to turn off the plot when you're graphing the function. If you want to graph both the plot and the function, correct the
dimension mismatch by making the number of x- and y-values the same. 
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(10)  I get an error message on my TI-89 Titanium when doing IRR.  Why?
       1)  If the first and last cash flows have opposite signs, you should get an IRR without problems.
       2)  If they do not have opposite signs, you may have problems according to the following:
           a)  If the cash flows all have negative signs, there will be no IRR.
           b)  If there are relatively small positive cash flows between negative first and last cash flows, you may
                either not obtain a solution or may get an invalid value. Of course, there's a vertical asymptote at
                -100%, but often you can run into problems considerably before you get there.In this situation,
                if you insist on solving this type problem you can graph it if the number of cash flows does not make
                the task too arduous.
                Very briefly, what you want to do is graph the following:
                Y1=C(0)+C(1)/(1+x)+C(2)/(1+x)²+C(1)/(1+x)³+...C(n)/(1+x)ⁿ
                 As a first try, set Xmax =2 and Xmin=-2.You can then find the value of the interest, X, using the
                 2ND, CALC, Zero, method discussed elsewhere in these documents. Let's take an example of
                 -500, 100, 200 , A, just to make the math simple.  As A becomes increasingly negative, the two
                 solutions move closer together so that at -63 the solutions about are -62% and -50.7%.  But if
                 A=-65 both of those solutions greater than -100% disappear and the calculator accurately gives
                 an error message.  Bottom line: If you plug in a negative number chosen at random, your calculator
                 is not malfunctioning if it gives an error message.
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(11)  When I multiply 360*$1258.59, I get 453092.  Why did it cut off the decimal.
       You most likely have the the FLOAT set on too few digits.  To change that, press MODE, move the curosr
        to "Display Digits," press the right arrow, select FLOAT 12 or whatever number you want, and press
        ENTER,

Revised: 8/14/09

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