PC-1211, PC-1250, etc. TVM

[dm319]

(06-05-2024 07:16 PM)robve Wrote:  2) B=1,N=40,PV=900,FV=-1000,PMT=-400
comp I% => 80% (exact)
comp FV => -1462.657745 (should be -1000, but TI BA gives -900(*) and HP prime gives -1001.6 so it's tricky to compute FV accurately)

This is a really curious example!

If you have FV = -100,000 what do you get for I%?

If you start receiving 900, immediately paying out 400, then you have 500. 80% of 500 is 400 interest, which means the balance is 900 at the next payment period, when you are paying out 400. Ad infinitum presumably.

You're paying off your interest, so it's odd that these calculators can return -1000 for FV.

It's also odd they can return you an interest rate no matter how large FV is, yet the FV can never get more than -900.

It's also quite fun to solve for N with PV = 900, PMT = -400, FV = 900. On my DM42 solver I get a div/0 error, on plus42 it correctly tells me data is invalid. But then put in FV = 1000. Plus42 returns and error, but my solver gives me a negative number.

Confused.