Re: LOG function or ln Message #4 Posted by Karl Schneider on 8 Apr 2008, 3:52 p.m., in response to message #1 by CaribbeanHP12c
"Caribbean12C" 
It appears that you're asking how to get the exact mathematical answer using the HP12C, rather than the answer rounded to the nearest integer that its standard TVM supplies.
I also got 31.20227 using an HP10B.
You're on the right track with logarithms, but the correct function is the natural logarithm denoted "LN" on the HP12C. ("LOG" denotes the base10 common logarithm on HP calculators.)
Solve algebraically the Compound Interest equation in Appendix D (p. 207) of the Owner's Handbook, for a noninteger value of n. You'll need LN to obtain the answer.
Added:
PV  PMT/i = (PMT/i)*[1+i]^{n} + FV*[1+i]^{n}
PV  PMT/i = [(PMT/i) + FV]*[1+i]^{n}
0  200/0.04 = [200/0.04 + (12000)]*[1+0.04]^{n}
(5000) / (5000  12000) = 1.04^{n}
5000 / 17000 = 1.04^{n}
n = ln(5/17) / ln(1.04)
n = 31.20227
 KS
Edited: 8 Apr 2008, 11:50 p.m. after one or more responses were posted
