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Given the formula (source) $$ M = P \times \frac{J}{1 - (1 + J)^{-N}}$$ Assume $P$ and $J$ remain constant. I want to be able to find $N$ for given $M.$

Could you please give an example of a numeric method (like secant to solve the same)? [My] problem is how to find initial 2 values for the secant formula.

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If I understood your question correctly, we're given, $M,P,J$ and we're trying to find $N.$

Well, we have $$ 1 - (1+J)^{-N} = \frac{PJ}{M} \\ (1+J)^{-N} = 1 - \frac{PJ}{M} $$ Take $\log$ both sides: $$ -N \log{(1+J)} = \log{(1 - \frac{PJ}{M})} \\ N = - \frac{\log{(1 - \frac{PJ}{M})}}{\log{(1+J)}} $$ Since $M, P, J$ are given, we can very well compute the RHS, and hence compute $N.$

Did I mis-understood your question?

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Of course, assuming $\dfrac{PJ}{M} < 1$ and $J > -1.$ – user2468 Mar 25 '12 at 5:47

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