Solving the TVM equation for the interest rate

[Albert Chan]

(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%

I made function guess_i(n, pv, pmt, fv), fitting TVM of i as quadratics

lua> guess_i(32,-6000,0,10000) * 100
1.609567691201156
lua> guess_i(36,13000,-372.53,0) * 100
0.16925628259544864
lua> guess_i(36,5750,-176.89,0) * 100
0.5625600411596099
lua> guess_i(360,75000,-425.84,0) * 100
0.4601141074876585