TI-89 NUMERIC SOLVER
by Donald Ingram
HOW TO GENERATE YOUR OWN FORMULAS, SAVE THEM, AND USE THEM.
Let us assume you want to use
the Numeric Solver application
of your TI-89. It is userfriendly, and a wonderful
problem solver. It can free you
from the difficulties and errors
of manipulating algebra. It can
find any unknown value simply
by your supplying the values
for the known variables. For
example, in an ohm’s law
formula, if you remember I = V
/ R, this application let you
solve for I, V, or R if you supply
the values of two of them.
Numeric Solver also preserves
your previous entries and
calculations and carries them
into new equations. It
automatically saves the last 11
equations you use on a stack,
ready for reuse. Numeric
Solver can also preserve your
equations in memory locations
and files which can have
significant names for your
recall.
Let us begin by entering some familiar formulas we often use in DC electronics. We will begin by turning on
the calculator and pressing APPS and then selecting Numeric So… .When Numeric
Solver is highlighted, press ENTER. There may already be an equation in Numeric
Solver, such as the voltage divider formula show here.
You see that whatever equation you have on your calculator, the whole equation is
initially highlighted. Press any key except ENTER or the down arrow, and you can
be entering the next equation. That’s convenience! So let’s enter the v=vs*r/rt
equation shown in this example.
Begin by pressing ALPHA v, =, ALPHA v, ALPHA s, x (multiply), ALPHA r, /, ALPHA r, ALPHA t.
In the future I will expect you to remember how to press characters like
you just did.
Press ENTER or the down arrow and the following screen appears:
In fact, there may already values after the equal marks, left there from
some previous use of these variables. By moving the cursor up or down,
you may enter the values you know. Lastly, move the cursor to the one
unknown value, and press F2. The calculator may pause a minute, but soon will produce the answer for the
unknown variable.
For instance, suppose we know that there are 3 resistances in series having
values of 47k, 22k, and 15k. We know that there are 6v across all 3
resistances, and we want to know how many volts are across the 22k. The
voltage divider formula can easily solve this problem.
v is your unknown, vs is 6v, r is 22k, and rt, the total resistance is 47k + 22k + 15k. k is EE3, and appears as E3 .
Enter the values as shown:
Notice that when you enter 22EE3 it appears as 22000 when you move the cursor.
Likewise, when you move the cursor to v=, 47E3+22E3+15E3 will all add together
and appear as shown. Notice also that I recommend you let the calculator do the
job of adding the three resistances, to reduce the opportunities for error.
Moving the cursor to v= and pressing F2, the calculator momentarily shows BUSY
in the lower right corner, then produces the voltage. You should round it to
1.57v or 1.6v, depending on the accuracy of your least accurate data.
We like this formula. Let’s keep it. Let’s save it in a folder called dc_elect, as a variable named v_dividr.
HOW TO CREATE A FOLDER
Press 2ND VAR-LINK (the – minus key) . Press F1 Manage. Select 5: Create Folder.
Press ENTER.
Key dc_elect. (to get the underscore _ you key the yellow
diamond key, then the MODE key.)
Press ENTER twice. dc_elect folder has been created. Return to the Numeric
Solver screen. If you press the escape key ESC, you should return to the Numeric
Solver screen. Your voltage divider formula should still be there! Now you want to save the voltage divider
formula, and any other dc electronics formulas that you may find useful.
In the Numeric Solver screen, press F1 Tools, and highlight 2:Save Copy As…
You may see main as the folder name, but press
your right arrow  and highlight your new dc_elect
folder. Press ENTER.
Press the down arrow .
Key the Variable name v_dividr and press ENTER
twice. You have now saved your voltage divider
formula forever!
Let’s try that again, this time with a parallel resistance formula. Key in a parallel
resistance formula into Numeric Solver. Here is an example for 3 parallel
resistors. You will notice that I used rr1 instead of r1. r1, r2, etc. are reserved
words and cannot be used. That’s why I chose rr1, rr2, etc.
Did you notice that your previous rt (resistance total) value automatically appeared? Once you are satisfied
that this formula works properly, use F1 Tools to Save Copy As rt_parll or perhaps r_parlel. Then you can go
on to save ohm’s law i_v_r and the three power laws p_i_v, p_i^2_r, p_v^2_r, and perhaps the current
divider i_dividr for parallel currents.
The rules for Folder names and Variable names is they must begin with a letter, must have no more than 8
characters (all lower case), and may contain underscore and numbers, but no spaces. oicu812 is valid, but
0icu812 is not. [Old joke, goes something like this: I just ate a hot pepper, and it really burned me! oicu812!]
HOW TO ACCESS A SAVED FORMULA
Later you will want to access one or more of the formulas you have saved. Simple. Just go to Numeric
Solver, press F1 Tools, highlight Open, ENTER.
The Folder may default to main, but you want dc_elect,
so just right arrow over and down until you highlight
dc_elect. Press ENTER.
Now the Variables will be arranged in
alphabetic order, lowest at top.
Press the right arrow and down arrow until you
highlight the one you want.
Then press ENTER twice. You got it, right there into the Numeric Solver screen.
Now let me illustrate a few features of the Numeric Solver which will put you miles ahead of an ordinary
scientific calculator.
We always give students problems like finding the voltage across one of a series of resistors, by using the
voltage divider. Did you ever see a problem requiring you to find the total resistance needed to develop a
specific voltage across a resistor? We know v=vs*r/rt. But what if we know the source voltage is 12v, the
required voltage drop across a 1k resistor is 3.5v, we just don’t know how much more resistance to add to it
in series? In Numeric Solver, I modified the formula to be v=vs*r/(r+rest), where rest is the rest of the
resistance needed for the total resistance in series. Enter v=3.5, vs=12, r=1000, place the cursor at rest= and
press F2. (Answer: 2429 ohms)
Or how about this one: we always calculate the resulting resistance of a number of resistors in parallel. But
what if the real problem is to create a total parallel resistance of a specific value, by adding a resistance
parallel to the others? Let’s say you need to make a 440 ohm total parallel resistance, and you already have a
2k and a 1k in parallel. What should the remaining resistor be? Try that on your TI-84! Well, with a little
manipulation of algebra, you may do it. On the TI-89, you simple pull up the r_parlel formula, enter r=440,
rr1=2000, rr2=1000, and go to rr3 and press F2. (Answer: 1294 ohms)
(Please note that not every total resistance can be achieved by adding another resistor in parallel.
Remember that the total parallel resistance must be smaller than the smallest parallel resistor.)