Generally, mortgage and annuity equations do not have analytical solutions for unknown interest rates, which have to be founds using numerical methods.  Another issue is that these equations have multiple solutions.  We discovered an interesting property of mortgage and annuity functions that can be used for computing the interest rate.  These functions have a single minimum and one or no inflection point, while the value of interest rate that corresponds to the minimum is equal to approximately one-half of the value of the correct solution.