Parlor tricks with TI-8x calculators

No, it's rhetorical. Don't bother answering.

Here's one to start off. This trick relies on the finite granularity of the TI family. First, let's get some simple algebra straight:

     x + 69 - x = y

y will always be 69, right? If you assign some random value for x and enter the expression on the left in your calculator, it will indeed come out to 69. Unless...

...you assign x to be some arbitrarily large number, e.g. 10^99. If you do this, and enter the expression into your calculator, you'll find that y = 0. What gives?

The enormous value of x is stealing the thunder of the constant 69. As the TI only deals with the 14 or 15 most significant digits of the value at hand, everything smaller is thrown out with the bath water.

I know for many of you this is probably petty and less than amazing, but I remember getting a kick out of it back in the day and thought I'd share.

A trick that will probably amaze quite a few is to seed the random number generator in order to get the results you want from tossing dice.

Following is my TI-83 source code for how to find a seed that gives me 5 ones on a six sided die in a row so that I could show my friend that I have Ba'alzamon's luck.

:0->P
:0->K
:1/6->G
:While (K=0)
:P+1->P
:P->rand
:(rand<G)(rand<G)(rand<G)(rand<G)(rand<G)->K
:End
:disp P

If rand<1/6 then randInt(1,6) will give 1. Now, run the program and take a lunch break. When it finds a result (my TI-83 finds the seed value 16086. Other versions might use other random number algorithms and will give different results) type in P->rand and clear the screen. Now show your friend that randInt(1,6) gives 5 ones in a row. (Actually on my calculator it gives 6 ones. This is only a coincidence, though.)

On the TI-83+, the calculator of choice at my school, there are several ways to scare people who let you touch their calculator:

This one doesn't work all the time, and has varying results from clearing the RAM to displaying lots of funny characters and breaking in a very stylistically way and clearing the RAM. The results vary, but it can be quite entertaining:

Press the following buttons:
Y=
Clear
Graph
Y=
0

Enter the following:
Equ-Str> (Find this in Catalog (2nd+0))
Y1 (Vars, Y-VARS, Function)
a comma (",")
Str1 (Vars, String)
Then hit enter a bunch of times. Then try accessing different screens like Graph and stats. Everything you cared about was archived, right?

Some fun Asm programs and how to make them:

Hit "Prgm", and make a new program, name it whatever you want The first line must be the AsmPrgm token (Find it at the top of catalog) Then enter the rest of the program, and to run it quit out of the program editor and enter Asm(prgmNAME). (Get the Asm( token from the catalog as well, and then for prgmNAME simply select your program from the Exec menu of the program list)

A program to "test" your LCD and make it display blue lines (by overheating the display)
AsmPrgm
3E1F
D310
C9
Wheeeee pull out the batteries to quit it.

A program that sets both the horizontal AND vertical graph split modes:
AsmPrgm
FDCB14C6
FDCB14CE
C9
Go to the mode menu to fix the problem.

Set the program to self-test mode:
Go to the mode menu and type "S"

And of course the classic standby to mess people up: Change their contrast all the way up or down (2nd+up or down arrows a bunch).

Another trick that works with most graphing calculators (probably scientific calculators too, though I don't have any to test) deals with non-integer factorials. I stumbled onto this after getting bored in math and trying out non-integer factorials. If you do (0.5)!, you get Sqrt(Pi/2). (-0.5)! gives you Sqrt(Pi). It's a pretty bizarre thing to see, especially as the factorial operation is technically only defined for whole numbers. It's especially fun to show to your math teacher (mine actually stopped teaching for the rest of the period to try and work out why this happens).

Thanks to MathWorld and an excess of free time, I found out a few days later that it was because most calculators use the Gamma Function of (n+1) to find the factorial of n rather than the recursive definition one would expect.

I successfully tested this on the following graphing calculators: TI-83, TI-83+, TI-83+ Silver, TI-86, TI-89, HP-49G+.