Solving the TVM equation for the interest rate

[Erwin]

(04-15-2018 08:48 PM)Carsen Wrote:  Problem #1: n=32 PV=-6,000 FV=10,000 PMT=0 i=???
Answers: 12C=1.609139492% 15C=1.609139501% Prime=1.60913949196%

Problem #2: n=36 PV=13,000 PMT=-372.53 FV=0 i=???
Answers: 12C=0.1692576094% 15C=0.1692569782% Prime=0.169257604168%

Problem #3: n=36 PV=5,750 PMT=-176.89 FV=0 i=???
Answers: 12C=0.56266017270% 15C=0.5626013555% Prime=0.562601726101%

Problem #4: n=360 PV=75,000 PMT=-425.84 FV-0 i=???
Answers: 12C=0.458302324% 15C=0.4583302039% Prime=0.458330232637%

The 12C results is from my 12C bought in 2016. The 15C results is your (Dieter's) program. The Prime's results is using the TVM formula in the 12C manual using your initial estimate formula. Looking at the 15C and the Prime's results, I would say that the estimate works really well. I wonder if there is a even better way to produce a estimate.

Hello,

cause I was curious I tried the above examples with the finance pac from the HP-71b with the following solutions:

Problem #1: i= 1.609139492% same as the 12c
Problem #2: i= 0.169257426%
Problem #3: i= 0.562601665%
Problem #4: i= 0.458330232%

So it looks like, that the HP-71b routine for small interest rates use a different method compared to the other calculators. But no idea what calculation method the HP-71b use, there are no hints in the user manual.

regards Erwin