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!