TVM formula error in programming manual?

[Albert Chan]

(10-30-2023 09:36 AM)dm319 Wrote:  If you look at page 92 in the HP-42s programming manual, it has the 'PV' term as '-PV'.

Yes, it is a typo.
Beware, some calculators (e.g. TI BA-35 Solar) do use this loan convention.

If compounding effect is high, solve NPV=0 for i is not ideal.
Same for NFV=0, since TVM is time symmetrical, NPV(n,i,pv,pmt,fv) == NFV(-n,i,fv,-pmt,pv)

It is better to pick a middle ground, a straighter curve.

Let C = i*n/(1-(1+i)^-n)      → Let NPMT = C*NPV

(07-11-2022 09:08 PM)Albert Chan Wrote:  NPMT = C*pv + C(n=-n)*fv + n*pmt

If time flow backward, we get back same NPMT

C(n=-n)*fv + C*pv + (-n)*(-pmt) = NPMT

(07-11-2022 09:08 PM)Albert Chan Wrote:  NPMT tends to be more "straight" than (NPV, NFV), with values inside (NPV, NFV)
Thus, f = NPMT/n = 0 is great for solving rate x, using Newton's method.

Newton's method for f=0 is the algorithm used in Plus42 TVM, to solve for i.
see TVM solve for interest rate, revisited