def f(tvm,i):
            '''function used in Newton's method; pmt(i)-pmt'''
            a = (1+i/100)**self.__n-1
            b = 100/i
            out = -(tvm.__fv+tvm.__pv*(a+1))/(a*b)-tvm.__pmt
            return out
function used in Newton's method; pmt(i)-pmt
recipes/Python/302697_time_value_money_class_demonstrating/recipe-302697.py