Version 4.7.4 MiniWinCP Demo 5.0 PolyRoot 1.04 68K PolyRoot 1.04 Carbon PolyRoot 1.04 PPC RPN Calculator 1.5.8 Carbon RPN. Where the second line is extracting the real roots of the polynomial equation in terms of (1+i), and the one you want is the one greater than 1 (1.0427750), from which you subtract 1.

You can also use more general internal rate of return functions for this problem, by supplying the vector of cash flows corresponding to the annuity. You're correct that FinCal::discount.rate does essentially the same thing as RATE, but it looks like it rounds all its rates off to three significant figures without saying anything about it, even in the help as far as I can see. The a P ( calc-poly-roots ) roots command uses H a S to solve an equation in general form, then, for all arbitrary-sign variables like s1, and all. If greater accuracy matters to you, that could be a problem.

(An alternative for more accuracy is to take the output of that discount.rate function and take one step of Newton-Raphson, which should do much better.

Polyroots calculator