Author: Anders C. Madsen (andersm@post3.tele.dk), Oct. 1996
Consider this material public domain. Comments, suggestions etc. welcome.
Tested on TI80, should run on any TI8X.
Being a poor student with the need for some TVM-calculations and without money enough to buy a TI-83 , I decided to "learn" my humble TI80 how to deal with these matters. Now that the work is done, I thought I'd share it -- you should be able to run it on any TI-8X, but I suppose that using SOLVE with the original equation would be somewhat easier. These routines uses 4 variables:
P = PV (present value)
M = PMT (payment)
N = N (number of periods)
I = I or IRR (interest)
PROGRAM:PV
:(M/I)-(M(I+1)^-N/I)->P
PROGRAM:PMT
:(I(P((I+1)^N)))/(((I+1)^N)-1)->M
PROGRAM:NPER
:(LN -(M/(IP-M)))/(LN (I+1))->N
PROGRAM:IRR
:0.1->I Basic value for I:0.1->Z
Initial increment value for I
:P->V Store the original PV-value
:LBL A
:PRGM_PV Find PV for I=0.0001
:IF PI otherwise we restore I to its previous value and
:Z/10->Z divides Z with 10 to increase the accuracy.
:END End of "IF PI PV(I) is higher than the original PV, so IRR
is still to low - we increase it with Z and
:GOTO A go back to A and give it another try.
:LBL B The correct IRR is now somewhere between I and I+Z,
:V->P so we restores P to its original value,
:DISP I displays IRR and
:STOP ends the program.
How to use it:
You go to your bank and borrow $10.000. The interest is 10% per rate and you repay the loan over 20 rates. How much do you pay per rate?
10000->P
0.10->I
20->N
PRGM_PMT
1174.596248 -- that is, approx. $1.175 per rate.
You buy a new stereo -- $1.000 -- and the salesman makes you an offer: Only 12 easy rates, $99 per month... it's a bargain! Hmmm...what's the actual interest paid?
1000->P
12->N
99->M
PRGM_IRR
.0276 -- he is charging you 2.76% per month!